We typically see around 2 weeks on average when it comes to coil life. There are quite a few things that determine this though which can alter the life span of a coil. For example the sweeter your E-Liquid is the quicker the coil usually goes out as the sweetener is what is caramelizing on the coil preventing it from burning properly resulting in a bad coil. Wattage can also play a factor depending on if you are firing the coil at the recommended wattage, normally I like to stay a bit under the max wattage for a coil in order to help preserve its lifespan.
You usually can tell when a coil is going out by the way it hits and tastes, and the flavor it gives might start to fade, this is usually a indicator that the coil may be on its last leg, and when it really taste burnt then you know it is time to go ahead and change the coil, usually you get a spare with most tanks and kits you buy these days.