No. There isn't.

Chain wear is not a matter of mileage : how many watts does the team put on the chains, how well it is maintained, is the bike ridden in the rain, etc. The timing will have a longer wear then the back chain but there is no way to tell how much longer without measuring it.

One should always measure his chains and replace them when they are worn out. By doing so you make sure 1° the chain won't break, 2° it will work better : with wear, the fits less and less on the chainrings and cogs and wears them. This wear is caused by friction and this means degradation of the bike's riding qualities.

lhbernhardt, you are quite right to say it's bad to re-rivet a chain -- unless a quick-link (such as Sram's PowerLink) or a connecting pin (such as Shimano's) is used. Your 5-6 spd chain will not sit properly on your 8-9 spd chainrings -- I presume this is what your bike is equiped with).