my longest so far was the first round of a tournament a couple weeks ago -- 2-6, 7-6 (7-2), 7-6 (7-4) in 2 hours, 45 minutes. My fitness saved the day as I made a boatload of unforced errors but managed to keep my energy level up and wear my opponent down.
What's funny is that last weekend I watched a little bit of Nadal v. Coria, went and played for a couple hours, ran some errands, and came back to see them in the 5th set. I thought 2 hrs. 45 min. was long and they doubled it.