I didn't have a lot of time to mine last night, but in three deposits I got 208, 170 and 266 total saltpeter from each deposit.
While mining, a block of Niter Deposit may appear next to where you are mining. When you mine the niter deposit, you will get zero, 1, 2, 3 or 4 saltpeter from each mining attempt until the deposit runs out. It starts out as a "Gigantic Niter Deposit," then goes to "Massive," "Huge," "Large," "Medium" then "Small" deposit. (I think the Medium is in there). The deposit disappears quickly once it gets to the "Small" stage. This seems a nice way to stop script mining of the deposit, but it sure takes a lot of clicking.
Luck is supposed to play a part in finding the Niter Deposit, but I am not sure whether luck plays any part in how many saltpeter you receive when mining the deposit itself.