Depends on the flavoring you use. I've had mine last as long as 8 days and as short as needing changed on the second day. Replacements heads are easy and cleaning them is pretty easy too. But, I'm having trouble with it randomly deciding to drain itself of liquid....
I mainly go by size. I don't want to refill the tank all the time regardless of how easy it is. That's why I got the PTII.
I think I'll get more than one new type of clearo and see how I get on with them. The liquid draining sounds like a PITA though...
I hear you about the size but like Optimo says the big ones will look stupid on my EVOD and I take it out and about a lot.