How much is reasonable for a hobbyist to spend on camera gear?

If you are married, or share finances, as much as you can syphon out of the account without having to sleep on the couch.

If you are single, as much as you want!

I don't think there is a hard a fast rule. Spend as much as you want, it's a hobby so enjoy it. Chances are you'll end up buying and selling all of your stuff time and time again. I started with Sony. A700 then A900, then they went to EVF which I didn't like, so I swapped entire systems to Nikon (D800) plus a slew of lenses. Got the okay from the wife, so I did it all in one shot.