I use bontrager race lites which go between 3500 - 5000 miles average which for the riding I do, is a bit more than most people get out of them. When they start to look worn or have too many super blue spots on them I will if possible) replace the front one first, wear it in for about 500 miles and then put a brand new one on the front and swithc the front tire to the back. But it also depends a lot on what my riding conditions are. I would not hesitate to replace them both at the same time if neccesary. I will add that I am very careful to check my tires after each ride and will replace a tire rather than have to change a flat tire. I also tend to keep them inflated to 115 psi which is just below the max 120.

Marni ( currently in Santa Barbara for double Ph.d graduation and wedding but no biking