It takes 2-3 minutes for an experiment to warm up. If you are paying attention, you usually can tell it's not going well at that point and cancel it. If it looks like it might go well, it can take around 10 minutes to settle. I've seen a few cases where some people got great results by letting it run for hours. But, with long runs I've also seen more cases where the evolution "gets bored" with where it is settling and makes a suddenly makes a large jump that rarely looks better than what it abandoned.