There are several factors that come into play the most important one would be our inability to estimate the quality of your methamphetamine. This can be only answered by you through trial and error of the product.
Methamphetamine has a pretty high bioavailability insufflated and given the product is sound you can get anywhere between 2-3 doses for a moderate tolerance. Still though this is a rough estimate and I would advise you with the maximum and work your way up to a minimum if need be.
Depends on tolerance, purity and route of administration. You already answered your own question so I assume you are just curious how much others use, which is pretty pointless given the aforementioned variables.