It’s not just hard to judge - it’s a moving target. 5GB will be enough, until you have accumulated enough uploads to use it all. If that’s a week, then of course 5G is not enough. But if it’s a year, or two years, is that enough? It depends on what timescale you are thinking of.
So, it’s about how many active users you accumulate, and how many images and videos you end up hosting. Which very much depends on the kinds of discussions you will have.
As a datapoint, my forum is 7 years old, has 959 users, 20 active users in last 30 days, 2700 topics, 260 posts in last 30 days, and a backup is about a gigabyte, 90% of which is uploads. Other forums will be very different.