Now many registered users is not really a factor...
Things that are factors are things like:
- Number of visits
- Now well things are optimized in general
- The number and size of images
- Forum hosted Avatars, Sigs, and Attachments
However, I would say that a well optimized and bare site which gets 100,000 visits a month could probably stay under 100,000MB of bandwidth. However, that doesn't mean that you could get 100,000 users a month and stay on shared hosting... In fact chances are you would need a VPS or Dedicated server...