There is no "set" period of when to change coils, really. I mean, different coil materials and gauges and builds have different amounts of robustness, and typically if you can dry burn a coil to nearly absolutely clean, they don't HAVE to be changed frequently. I mean, people do, and I will admit to thinking there MUST be a certain amount of masochism involved, but that's me.
If you aren't trying new builds and stuff (which is a perfectly valid reason to change a coil) well, some folks get quite a bit of life out of their coils, like months sometimes but as stated above it depends on a variety of factors. Juice flavor and whatnot does matter, IMHO more to the wick than the coil, but the more gunked a coil gets, the harder it will be to get clean, even if you include stuff like scrubbing it with a toothbrush.
I just go by "Is it still vaping fine and is it something I would be ashamed to post on ECF" standards, and I'd say my coils are sort of middling, some like 3 week, some 7 weeks or longer.
The coil type and its robustness really do matter, too.
Anna