Batteries are generally rated for about 300 charges, but the best way to tell is by your own observation. When you notice it's not holding a charge it's time to replace it.
How long a charge lasts depends on your vaping style. Low ohms/high watts vaping will drain the charge more quickly. The mAh listed on the side of the battery is the milliampere hours it's rated for. Technically, 100mAh is roughly equal to 1 hour of use - as in 800mAh would be 8 hours of use, but with the variable power used in vaping it is less. My main rig is a 1.5Ω RTA at 4.4v/13w. I expect to get about 4-5 hours out of an 800mAh 18350 battery.
ETA: I see now that you were asking about an 18650 battery. I still knock off about half the listed mAh. If it's lasts longer I'm happy, if it doesn't, I'm not surprised.