Bandwidth is something that comes with any web hosting account. Bandwidth measures how much traffic you are allowed to get. Bandwidth is used every time a page is loaded on that host account. Usually a regular used website will need about 10 gigabytes of bandwidth per month. Larger websites can use up to several terabytes of bandwidth a day.
If someone is looking at one page for 30 minutes, the person is NOT using any bandwidth so you can’t assume that. Bandwidth was only used when the page was loaded at first. And if you want to open a website soon, I would recommend that you start off with at least 1 gigabyte of bandwidth just in case. If you run out of bandwidth, your websites will no longer work until the next month, try to avoid that.