I really don't understand how people can get through so much ammo, I only top up once or twice a week. Pick you target, use an appropriate weapon for the range, and fire short bursts, not just squeeze the trigger and hope.
I honestly have not purchased ammo for at least 2 months since i clicked "Purchase all ammo" by mistake trying to fill up my newly aquired sniper. Hell... I dont even buy armor.
I agree with you that you can go quite some time without purchasing ammo if you fill up all in one go but it also depends what games modes you do the most or what you do in free roam. In free roam I hardly use my gun and I save ammo that way and as for game modes I'm normally in Co-op mission or race so again I use very little ammo here. So it all depends on what games modes and how you play the game depending on how much ammo you will use.