Iride01 

I don't have any set time or mileage to check or replace. When I feel so motivated, I check mine with the typical chain checker from Park Tool, but at times I also will measure with a metal scale. If either shows more than .5% or a 1/16th longer than 12 inches, I start mulling over changing it.

I'm not much for preventative maintenance anymore. I think it cost more money in the long run when you get too finicky about things. Sure, if my bike was soaring regularly at altitudes above the ground higher than I'd want to fall, I'd be all up in the preventative maintenance thing.

So if I happen to go too long with my chain and the cogs and chainrings get worn to badly, I can replace them too for not much money. Probably less than number of chains I would have had to replace early to save them from extra wear.

But that is not to say you shouldn't replace early or on some time schedule. It's up to you and your circumstances. Also depends a lot on whether your mindset is someone that like to tinker and fiddle with mechanical things to perfection. I used to have that mindset, but lost that as I got older. It's more enjoyable to ride than it is to prepare everything to ride.

As far as mileage on chains, I would have sworn I was getting 8000 miles out of a chain. But those were all on bikes less than 9 speeds with a 14 tooth cog the smallest. My 11 speed chain is ready for replacement now with just 4500 - 5000 miles on it.