I tested 3 different primary weapons and 1 secondary weapon.

Saiga: Clip size 7, capacity 70: Four full reloads (4x70 ammo) before ammo bag depleted.
.308 rifle: Clip size 10, capacity 40: Four full reloads (4x40 ammo) before ammo bag depleted
AUG: Clip size 30, capacity 120: Four full reloads (4x120 ammo) before ammo bag depleted

MP5 SMG: Clip size 30, capacity 150: Four full reloads (4x150 ammo) before ammo bag depleted

If you haven't noticed a pattern here, one full reload of an empty weapon consumes 1/4 of the ammo bag. Number of clips and number of bullets/shells did not seem to matter at all. When I tested both weapons at the same time, however...

AUG + MP5 SMG: Clip size: 10 + 30, capacity: 40 + 150, 2 full reloads (2x40, 2x150 ammo) before ammo bag depleted.

In other words, every weapon when fully depleted will consume 1/4th of the ammo bag. If both are depleted when the ammo bag is used, it will consume 1/2 of the ammo bag. I've noticed in games that I've played that the game will prioritize your secondary weapon's ammo over your primary weapon's ammo (if an insufficient amount of the bag is left to fill both).

Thus my final conclusion: Try to only refill the weapon you want to use for the majority of the time. This will make the ammo bag last longer.