I've run a build for a month at the most. Dual 24's @ .18. I never had any problems doing that. The only real issue I can speak of is that once they gunk-up once or twice, it's like pregnancy. It happens more easily after the first time. Personally, I start to notice a drop-off in flavor after just a couple of cleanings. After cleaning, it'll be good for a little while, but once the gunk sets back in, the flavor dies.
You can probably run a coil for as long as you want. Just watch out for obvious signs. If you start seeing orange rust or they're losing their shape, then they're starting to really degrade and they should probably go. At that point, it's anybody's guess what exactly it is that you're inhaling. When this happens is going to depend on how hot you run them and how thick the wire is, but when it really can't wait, you'll know.
I'm hooked on that fresh-coil flavor, myself. Nothing like a vape off of brand new coils... ...but then, I'm one of those people who gets the itch to change builds every other day, so it's just routine for me to change them often. It is that way because I really enjoy building coils. If I don't build at all for one day, I feel all wrong the next day. If I didn't like doing it, I would probably be pretty content to vape on the same set of coils for week after week.