Very much depends on use... if the bike is an errand hopper, it will stretch the chain faster.  If most rides are 25+ miles, it will last longer...
(cold oil doesn't circulate as well as warm)...

Generally best to check at around 8 to 14k.

I didn't do mine until 20k,.. but it is ridden on plenty of longer rides...
I had one that I bought that I thought seemed abused, so I checked it, and it was in need at 4k...
You never know with a used bike...