I replace the coils on my Evods and Protanks about every 6 days or so. Dry burning just didn't work for me - always had a burnt taste afterward. Don't want to get into re-wicking/re-coiling. I figure for a box of coils at around $6 it's still cheap enough. I've been using the same 2 or 3 tanks forever. Sometimes I just give them a good rinse out; sometimes I just add juice as needed.
Re-coiling / re-wicking is actually very easy. Search youtube for instructional videos if you want to give it a try!!