What
@RMarcusY said - darker and/or sweeter liquids tend to gunk up coils quicker. The number of days you've had your coil doesn't matter so much as how much juice you're running through it - I've had some juices that gunked last-generation coils like those for the Nautilus in as few as 3-4 tanks, though I could usually get a dozen or more.
If you're really liking the gunky juice and the cost of coils is getting to you, you might want to look into either rebuilding your stock coils or getting a device that's meant to be rebuilt - either choice will require some more work, but will dramatically drive the price down. Failing that, look for lighter juices that aren't as heavy on the sweeteners - and as mentioned above, you probably don't want to go above 40/60, maybe 30/70 if you don't chain vape, on the PG/VG ratio for the tank you're using, as thicker juices tend to wick slower and with the little wicks on those replaceable clearomizer heads you run the risk of getting burnt, dry hits. Your bottle may have the PG/VG contents labeled, if not ask your shop next time you're in - if their staff is good for anything they should be able to tell you.