diff --git a/docs/api/config/testTimeout.md b/docs/api/config/testTimeout.md
index 1ed253406..494433889 100644
--- a/docs/api/config/testTimeout.md
+++ b/docs/api/config/testTimeout.md
@@ -9,7 +9,7 @@ redirect_from:
 version_added: "1.0.0"
 ---
 
-Set a global default timeout in milliseconds after which a test will fail. This helps to detect async tests that are broken, and prevents tests from running indefinitely.
+Set a global default timeout in milliseconds after which a test will fail. This helps to detect async tests that are broken, and prevents a test run from hanging indefinitely.
 
 <table>
 <tr>
@@ -22,4 +22,44 @@ Set a global default timeout in milliseconds after which a test will fail. This
 </tr>
 </table>
 
-This can be overridden on a per-test basis via [assert.timeout()](../assert/timeout.md). If you don't have per-test overrides, it is recommended to set this to a relatively high value (e.g. `30000` for 30 seconds) to avoid intermittent test failures from unrelated delays one might in a browser or CI service.
+This can be overridden on a per-test basis via [assert.timeout()](../assert/timeout.md).
+
+It is recommended to keep the global default at `3000` (3 seconds) or higher, to avoid intermittent test failures from unrelated delays that may periodically occur inside a browser or CI service.
+
+### Deprecated: No timeout set
+
+Starting in QUnit 2.21, a deprecation warning will be logged if a test takes longer than 3 seconds, when there is no timeout set.
+
+```
+Test {name} took longer than 3000ms, but no timeout was set.
+```
+
+You can prepare yourself for QUnit 3 when this happens, by either calling `assert.timeout()` inside those tests, or by setting `QUnit.config.testTimeout` once globally with a higher timeout.
+
+Depending on your test runner of choice, there may be more convenient ways to set configuration:
+
+* Set `qunit_config_testtimeout` via [preconfig](../config/index.md) as environment variables (for Node.js), or as global variables for HTML/browser environments (before QUnit is loaded).
+* Set `testTimeout` via [karma-qunit](https://github.com/karma-runner/karma-qunit/#readme):
+  ```
+  config.set({
+    frameworks: ['qunit'],
+    plugins: ['karma-qunit'],
+    client: {
+      qunit: {
+        testTimeout: 5000
+      }
+    }
+  });
+  ```
+
+* and, are not using a test runner that sets its own timeout (e.g. [grunt-contrib-qunit](https://github.com/gruntjs/grunt-contrib-qunit)),
+
+### Introducing a timeout
+
+Prior to QUnit 3, there has not been a default timeout. This meant that a test could time out or get stuck for many seconds or minutes before diagnostic details are presented (e.g. after a CI job reaches the maximum run time).
+
+QUnit 3.0 will change the default timeout from undefined (Infinity) to 3 seconds.
+
+### Changelog
+
+| UNRELEASED | Announce change of default from undefined to `3000`, with a deprecation warning.
diff --git a/src/assert.js b/src/assert.js
index 25c541eb3..94cc8e7f7 100644
--- a/src/assert.js
+++ b/src/assert.js
@@ -81,7 +81,7 @@ class Assert {
   // Alias of pushResult.
   push (result, actual, expected, message, negative) {
     Logger.warn('assert.push is deprecated and will be removed in QUnit 3.0.' +
-      ' Please use assert.pushResult instead (https://qunitjs.com/api/assert/pushResult).');
+      ' Please use assert.pushResult instead. https://qunitjs.com/api/assert/pushResult');
 
     const currentAssert = this instanceof Assert ? this : config.current.assert;
     return currentAssert.pushResult({
diff --git a/src/core.js b/src/core.js
index 3c9404122..8bf783b05 100644
--- a/src/core.js
+++ b/src/core.js
@@ -124,7 +124,7 @@ extend(QUnit, {
 
   load: function () {
     Logger.warn('QUnit.load is deprecated and will be removed in QUnit 3.0.' +
-      ' Refer to <https://qunitjs.com/api/QUnit/load/>.');
+      ' https://qunitjs.com/api/QUnit/load/');
 
     QUnit.autostart();
   },
diff --git a/src/core/config.js b/src/core/config.js
index 0387d69bd..4e34207a2 100644
--- a/src/core/config.js
+++ b/src/core/config.js
@@ -127,6 +127,7 @@ const config = {
   // started: 0,
 
   // Internal state
+  _deprecated_timeout_shown: false,
   blocking: true,
   callbacks: {},
   modules: [],
diff --git a/src/test.js b/src/test.js
index 880cd7fde..9cb68bf14 100644
--- a/src/test.js
+++ b/src/test.js
@@ -759,6 +759,18 @@ Test.prototype = {
           config.timeoutHandler(timeoutDuration),
           timeoutDuration
         );
+      } else {
+        clearTimeout(config.timeout);
+        config.timeout = setTimeout(
+          function () {
+            config.timeout = null;
+            if (!config._deprecated_timeout_shown) {
+              config._deprecated_timeout_shown = true;
+              Logger.warn(`Test "${test.testName}" took longer than 3000ms, but no timeout was set. Set QUnit.config.testTimeout or call assert.timeout() to avoid a timeout in QUnit 3. https://qunitjs.com/api/config/testTimeout/`);
+            }
+          },
+          3000
+        );
       }
     }
 
diff --git a/test/cli/cli-main.js b/test/cli/cli-main.js
index 314d58384..4e919002f 100644
--- a/test/cli/cli-main.js
+++ b/test/cli/cli-main.js
@@ -16,6 +16,7 @@ QUnit.module('CLI Main', () => {
     // and only await/assert the already-started command.
     concurrentMapKeys(readFixtures(FIXTURES_DIR), 0, (runFixture) => runFixture()),
     async (assert, fixture) => {
+      assert.timeout(10000);
       const result = await fixture;
       assert.equal(result.snapshot, result.expected, result.name);
     }
diff --git a/test/cli/fixtures/config-testTimeout-deprecated.js b/test/cli/fixtures/config-testTimeout-deprecated.js
new file mode 100644
index 000000000..8f7147bd9
--- /dev/null
+++ b/test/cli/fixtures/config-testTimeout-deprecated.js
@@ -0,0 +1,17 @@
+QUnit.test('fast async', function (assert) {
+  var done = assert.async();
+  assert.true(true);
+  setTimeout(done, 7);
+});
+
+QUnit.test('slow async 1', function (assert) {
+  var done = assert.async();
+  assert.true(true);
+  setTimeout(done, 3500);
+});
+
+QUnit.test('slow async 2', function (assert) {
+  var done = assert.async();
+  assert.true(true);
+  setTimeout(done, 3500);
+});
diff --git a/test/cli/fixtures/config-testTimeout-deprecated.tap.txt b/test/cli/fixtures/config-testTimeout-deprecated.tap.txt
new file mode 100644
index 000000000..73c3189f3
--- /dev/null
+++ b/test/cli/fixtures/config-testTimeout-deprecated.tap.txt
@@ -0,0 +1,14 @@
+# command: ["qunit", "config-testTimeout-deprecated.js"]
+
+TAP version 13
+ok 1 fast async
+ok 2 slow async 1
+ok 3 slow async 2
+1..3
+# pass 3
+# skip 0
+# todo 0
+# fail 0
+
+# stderr
+Test "slow async 1" took longer than 3000ms, but no timeout was set. Set QUnit.config.testTimeout or call assert.timeout() to avoid a timeout in QUnit 3. https://qunitjs.com/api/config/testTimeout/
diff --git a/test/cli/fixtures/hanging-test.tap.txt b/test/cli/fixtures/hanging-test.tap.txt
index 449758e4a..a364f2c87 100644
--- a/test/cli/fixtures/hanging-test.tap.txt
+++ b/test/cli/fixtures/hanging-test.tap.txt
@@ -4,7 +4,8 @@
 TAP version 13
 
 # stderr
+Test "hanging" took longer than 3000ms, but no timeout was set. Set QUnit.config.testTimeout or call assert.timeout() to avoid a timeout in QUnit 3. https://qunitjs.com/api/config/testTimeout/
 Error: Process exited before tests finished running
 Last test to run (hanging) has an async hold. Ensure all assert.async() callbacks are invoked and Promises resolve. You should also set a standard timeout via QUnit.config.testTimeout.
 
-# exit code: 1
\ No newline at end of file
+# exit code: 1