The sweat will hurt the gun more than the ammo... Good carry ammo has sealant around the primers anyway.
That is part of the reason I carry a Glock; between the polymer and tennifer, there is really just about zero environmental condition it won't shrug right off. Prior to that I was carrying Sigs, and didn't like sweating on those "nicer" pistols, especially the 228 with the blued slide. That was rust waiting to happen. The nitron is better, but I still don't think it is as good as tennifer.
I worked mostly outdoors in Houston for one summer about 3 years ago (worst heat and humidity especially, that I have ever encountered anywhere), and had switched to the G19 and Comptac holsters by that time, and it was fine. Got a little bit of rust on some of the hardware on the holster, but it could be wiped off and didn't affect function. The belt, on the other hand (Beltman leather)... it was soaked through every day and basically ruined within that 3 month stretch. Got a Wilderness Instructor's with stiffener after that; it is still going strong. Based on all this, for carry gear, make mine with as much polymer and synthetic material as possible; thanks.
Ammo, being contained within the gun or mag, should not be directly subject to liquid sweat, but it is pretty durable anyway. I rotate mine every 6 months to a year or so and consider that schedule to be more than enough. I'm more worried about setback from repeated chamberings than I am about powder degradation from environmental conditions.