We've been on discourse since the end of Nov. 2016. Our uploads grow by about 0.5MB/month. Our old platform had an image upload facility, but it was a manually vetted process, so there was a delay until the photos were available (and the photos weren't inline in the forum posts, you had to post a link to the photo album). It's an antique car forum, so including a photo showing some detail is a boon, and the users love it - I would guess that 10-15% of the posts include a photo.
Reducing the max upload size (3072) isn't an option, as the pics need enough detail that they show... detail.
Limiting the user's ability to upload (either by group membership, or by MB/month) wouldn't really work either - if some new user posts a question of the form "Is this right?" the best way to do that is often to include a picture of the carb, or brake pipe, or whatever, and the easiest answer is that someone with the same car answers by uploading a picture in response. Often a post will start with a detail question and quickly garner 5 "here is how mine is" responses with photos
We're not using S3 for uploads yet, but it's only a matter of time. As the users master uploading photos, and start really using it our backup window will get longer. At the moment our backup is 5GB: 1.5GB of uploads, and 1.8 Million posts.