My off hand estimate would be around 20lbs. This one’s actually going to vary pretty wildly because the weight limit has more to do with how much you can lift and carry comfortably, than some hard limit. Also, there are plenty of firearms that are man portable, but require setup. The .50 Barret comes to mind, and that’s a 32lb gun.
Ideally, you want your rifle to weigh less than 10lbs, and your pistol to weigh less than 3lbs. This isn’t so much a human limit as what’s comfortable for most people to use easily.
Something that’s actually worth remembering is that the weight of the weapon actually works in your favor for controlling recoil. A heavier gun will have more mass, meaning perceived recoil will be lower. It’s a detail that can be difficult to appreciate until you’ve fired a variety of weapons.