It's hard to say I know at a glance because it totally depends on how much traffic you get. I always wondered though what is the average that most "active" forums need for hosting? Does the filesize of images you use affect bandwidth usage?
Media files like photos and video will always use more bandwidth than text. As far as how much bandwidth you will need will depend on how much traffic you will be receiving. If you are just starting out you can always go with a low tier plan like 10GB or even lower and scale up from there when you need it.
Obviously those kind of media files will affect your bandwidth more in compare of just text on website. The more images and videos you will use on your site it will eat more bandwidth. If you want to go with limited bandwidth plan then it is better to use limited images and other media files. Otherwise you can go for unlimited bandwidth plan if you are using more media files.
In a free site web hosting, the bandwidth is so declared. That a limited space is offered for free. For a bigger bandwidth, it would be turned or subscribed to a premium status. Then you can enjoy the unlimited bandwidth.