It really depends on what you do online. Just being logged in and browsing around wont take much but downloading large files, streaming netflix, etc. can take quite a chunk and should probably be avoided with your low cap. Playing games shouldn't take up
too much bandwidth but it might be worth checking if your ISP provided any sort of bandwidth meter so you can experiment with how long you play.
I get something like 40 gb for the past 5 years (Yes I have been a member that long) and there was a stage me and my bro would play it it for like 6 hours on a Saturday and we have never been over the limit or close to it so it does hardly take any bandwidth
I have AT&T and we have bandwidth cap. I can't quite remember what it is though. But if I ever actually to hit the cap I will switch ISPs in a heart beat. A bandwidth cap is complete garbage and ISPs shouldn't even have one.