I'm asking because I have a 5 gigabyte limit for internet usage. The bandwidth usage monitor on my computer says it's using a miniscule amount, which I don't beliebe, since the usage as been going up really fast recently, though it might not even be me. So does anyone know for sure how much data this game uses every hour or so?

It could vary due to a number of factors, such as if you're downloading mods or other players' custom folders.

I don't use any weird mods, only normal ones like Aikido that come with the game. That said, worst case scenario, how much could it suck up? When a player with a kiai shows up, the usage monitor seems to respond accordingly, but eh.

Downloading customs uses the most. If you're worried about it, lock up the custom folder to restrict write access (means everyone will be a Tori and an Uke). Or, disable "Body Textures" in the Setup menu, although I'm not sure if that stops them from downloading or if it just stops displaying them...

Mods, replays, and scripts are just text files and use hardly any bandwidth.