just make sure you get plenty of extra coils/heads so when you need them you have-um....some last for weeks when maintained properly and some go bad overnight, there are alot of factors involved.....when vapor production becomes noticeably different to you (less), leaking Tank (juice on your batt post), diff taste; it's time to change them out.....take your old ones and throw them in a pill bottle or something & soak and shake (distilled water) then let them dry...and save them for hard times, you might get a little more use out of them....