10gb space & 100gb Bandwith Is This Enough For A Mybb Forum

You can even measuring that by analysis your apache log files, like how many bytes are sending out one day?
I can guess that if you have 100GB bandwith, it can be used for a large forum with 10k uniq ip per day (just my guess work)

 

If you don't offer much download/upload for visitors, should be much.

normally, forums need a little traffic because it sends only message. anyway, if you have so much pictures on the forums, your quota may exceed sometimes. depends on your configurations then