I clean my coils and re wick when one of the following happens:
Flavor is off
There is visible build up on the coil
The cotton becomes dark
Ramp up time seems to increase
To clean, I pull the cotton out, get the coils to a light glow, and quench the coils with water. I usually repeat this a total of 2-3 times.
Usually 5 to 7 days for me. Up to 10 days with an extremely clean vaping juice. As little as 2-3 days with very sweet or custardy or dark juices.
As far as "needing" to change the coil, I change when the following happens:
It seems to gunk faster between cleanings
The resistance changes
Flavor seems off after a cleaning and re-wick.
Coils should last at least 1 month. Probably a good average seems to be 3 months. They will last longer if cleaned frequently. SS coils seem to last the longest. But also seem to gunk up quicker.
I like to build, so I usually do not make it to the end of a coil's life before replacing it with something new and exciting.
Dripping is the way to go if you like to build. Cannot remember the last time I even considered using a tank (even a RTA.)