If you're asking about Smart-Runner, then this feature is already built in.
On the first time you run a test, the Smart-Runner classifies it as either a fast test or a slow test.
Tests that are classified as slow(longer than 2 sec) will run for another 3 seconds.
After 5 seconds overall the runner will kill the test execution and continue to the next test.
The slow test will not be ran the next time you run all test.
A slow test session will be triggered either manually or when your machine is in Idle(5 minutes).
You can also put the don't-run attribute
in this case the test will be transparent to the Smart-Runner .