In a perfect world, we would always have unlimited refills, unlimited anytime minutes, and unlimited bandwidth. In reality these things are finite, and nothing lasts forever. Bandwidth is not horribly expensive, but if you have your own dedicated server, you are definitely paying for it. Anytime you offer hosting or other services to your clients, you will have to decide if you want to limit bandwidth consumption.
Many web hosts promise “unlimited bandwidth”, and while that can be a great way to draw in customers, you will still realistically have to limit them to some extent. Most users will never come close to using the full amount of allotted bandwidth. That means you will have more available for the heavy users. That is why it is important to monitor the usage of heavy users to make sure they are not abusing the privilege, especially with illegal activities.
As long as you are monitoring bandwidth and do not find anything suspicious, you will probably be OK and can continue to offer your customers “unmetered” bandwidth. This means that they are not technically limited by bandwidth restrictions within reason. As long as they do not abuse the privilege, you should be able to offer them more than enough.
There are many tools you can use for bandwidth monitoring on a Linux server, and your Linux distribution’s software repository likely has options you can easily install to get started.