This varies depending on usage.
Regular maintenance can greatly extend the lifespan of your coils. Depending on how large the concentrate you are loading into the atomizer, you may need to clean it after every few sessions, or every week or so.
When your hit starts to taste a bit funky or off, you know it's time to clean some of the gunk of your coil. You should be getting rid of most or all of that taste with your cleaning session.
If your coil is not getting clean anymore and that taste seems permanent, you know it's time to use a new coil.