Depends on your juice - flavor, PG/VG ratio - and vaping style.
I have a couple of dark thick one's that will gunk a head in 3 mls, thinner juices that don't gunk up for 9 or so.
At that time, you can, if you wish, toss them, but they're easy to clean and dry burn. Done properly, that makes them pretty close to brand new. I find I usually have to replace the flavor wicks after I dry burn. You pull the post off the top of the head and have to remove the flavor strands to see the coil, necessary to properly dry burn. Stock is two strands of 1 mm silica, but I just use 100% cotton yarn.
Similar rehab can be done on top coil units, of course.