It also depends on the juice you are using. With the vanilla juice I'm vaping, I had to change my coils on my Kanger Pro tank every 3-4 days. With a different juice it lasted 2 weeks.
In general, I could tell it was time, when the juice turned a lot darker, compared to the color of the liquid in the bottle. With the vanilla, it also developed a burnt flavor, when I waited too long on changing the coil.
On a Kayfun and the vanilla juice, I needed to do a dry burn (without the wick) and change the wick every week. With another juice, I had to do this procedure after 3 months. Be careful with the dry burns. I melted an insulation sitting on the positive pole. Luckily I noticed it on time, and that Kayfun was still working.
The coil itself, you'll have to watch during those dry burns. If there are hot spots, etc.