I think it is time to swap Clutch when he starts shutting down the boards to all those that haven't contributed to the tipjar. oh, wait, are you talking about cars?
Don't worry, it'll tell you when you are at a stop light in the middle of nowhere. Oh maybe that was my clutch - Just prior to the 400 dollar bill.
If it feels like mush, it could be your slave cylinder. Normally when you replace a clutch you do the slave cylinder at the same time, but last year I replaced my slave cylinder alone and left the clutch in, and it runs fine.
That was why my bill was so high. Some of the mechanical parts, need to be replaced simultaneously. Good point
Change it when the clutch starts slipping. You're engine will rev, but your car will not really go. Is it a hydraulic or cable clutch? On a cable clutch, make sure you change out the pressure plate and throwout bearing. If your rear main seal is leaking, it's a perfect time to get to that bad boy as well. A new pilot bearing is a good idea as well.
I agree with the above post. From my own experience it seems like the clutch starts going around 120K miles.
I won't replace my Clutch, ever. PERIOD. He does a good job of administering this site and I love him. PEACE, and DEATH to all CLUTCH replacers!