Bandwidth is offered to Steemit users in proportion to how much Steem Power they have. If a user has X amount of Steem Power then he will receive Y amount of bandwidth. If another user has 5*X amount of Steem power then his bandwidth will be Y times 5. 

When a user posts something on Steemit, he losses some Bandwidth for that action. Users with a lot of Steem Power generally won't have their bandwidth run out to zero. But new users with 2-3 Steem Power may notice that their Bandwidth is very low after posting a lot of comments or posts. 

Steem users can check their Bandwidth allocation by visiting steemd.com/@[username of that person]. As an example, I have shown my bandwidth in the following image. I have 5.4gb of bandwidth left. Even