Right now, I'm outside the US and my weapons are in storage, but in college I had what I considered a reasonable amount of ammo on hand. I kept 500-1000 rounds of .22 for my pistol. When I went out plinking I generally shot a couple hundred rounds, especially if I was taking some one shooting (great date by the way). For my 9mm CCW, I had 100-150 rounds. I mostly shot it for familiarization, say 50 rounds a session at the range, since I could pour rounds through the .22 to practice marksmanship and 9mm is more expensive. For the single shot shotgun I kept around 50 rounds of buck and bird shot, plus a 10-20 slug rounds. Its hard to go through a lot of ammo with a single shot weapon, and I didn't see a need for anymore then that.

When I get back to the US I'll probably end up with about the same amount of ammo. If I pick up a semi-auto rifle, I'll probably keep about 500 rounds on hand for it as well. I generally go off of how much ammo I shoot at the range in a single session and keep about three times that amount on hand fluctuating up to five. For something hard to find, I'd probably buy all I could get. For something as ubiquitous as 9mm or 12 gauge, I wouldn't worry about how much I had nearly as much as long as I had "some". I was also in college, didn't have a great place to store it and didn't have a lot of cash. The nice thing about .22 was 1000 rounds was $20.