Let's say you have a future async task that makes an external call and should ideally finish in under 2 seconds and you want to write a test for the same.

class MockClass {
Future<void> initialize() async {
    await Future.delayed(const Duration(seconds: 1));
    _isInitialized = true;
  }
}

Here's how you can do it

MockClass object = MockClass();
test('should be able to initialize in under 2 seconds', () async {
      await object.initialize(); // This is the function that should 
      expect(object.isInitialized, true);
    },
        timeout: const Timeout(
          Duration(seconds: 2),
        ));
  });

The output: ✓ Mock Authentication should be able to initialize in under 2 seconds

Note the Timeout class where you can set the duration.