I do it A LOT as I travel for work constantly and I'm always in the van.
I'd teather my phone to my laptop to chat or spectate friends.
I'd use around 400mb/hour less or more.. Depends really what you're doing. If you are just playing and chatting will probably be 200-400mb.. If you're spectating and downloading replays, will probably 400-800mb...
I'd be in the van for 3 hours and use up a gb no problem.