Surely you should know how long it lasts if you've been snorting it yourself?
Anyway, snorted, the effects usually last 1-2 hours.
As far as I can tell, mephedrone salts decompose before they get to a high enough temperature for vaporisation, and the freebase form is an unstable alkaline oil - either way, not smokable.
Edit: Actually, reading around, among those who have tried to smoke it in salt form, some are saying that it works (but apparently not very well?), others saying it doesn't work at all. People getting different salts, perhaps - who knows?