One guide I always found interesting comes from an old RPG set in Vietnam called "The Revised Recon" -
http://en.wikipedia.org/wiki/Recon_%...laying_game%29
In that (on pages 45 & 46 if you get hold of a copy) they give an example of the gear carried by a new arrival in Vietnam and what he's carrying six months later. It's an RPG so I've no idea of the level of accuracy but he goes from carrying 4 spare mags for his M16 to carrying 16 (and ditching a lot of other crap as well).
I think however that the key question for your game, and the amount of ammo the characters are carrying around, is how frequently they think that they're going to be resupplied? If they're not going to be in action for very long and can expect an extraction fairly quickly then they don't need to carry as much ammo. If they know that they're going to be on the ground with multiple firefights then they know they need to carry more.
However as a GM I'd give them some advice and then let them decide and see what happens, quite possibly putting them in longer range firefights that use up more ammo, particularly if there was a PC not carrying enough ammo and I wanted to make a point. Obviously should anyone start running short of ammo there is always the option of switching to a captured weapon.