The answer is usually some variation of "a lot."
Since we're nearing the tail end of the biggest ammunition shortage in recent memory, we now have a small bit of experience in dealing with poor resupply of ammunition.
I'll wager that none of us have depleted our apocalypse reserves--no, what suffered has been our ability to get out there and train. Blowing through a hundred rounds at the range has become cost prohibitive, let alone a full fledged 3 day weapons course with a reputable instructor, where round counts easily top 1000 rounds.
Personally, I am beginning to use a rough average round count from several schools as the baseline, then doubling it. This gives enough ammunition to run through a training course and still have lots of ammunition left in reserve. Also, should the zombies arise and the communists invade, it's a lot of ammo to begin with.
Here are some rough numbers this approach gives you - tending towards the higher counts:
- Semi-Auto Rifle: 3000 rounds
- Sidearm: 2400 rounds
- Shotgun: 1200 bird, 400 buck, 200 slug
- Precision Rifle: 1200 rounds match ammo
Obviously, at today's prices, this is a sizable investment.
The ammunition horde is one area that I personally need to improve on, and as ammo prices fall back to earth, I will be looking to make some headway here.