We keep 2 backups and use S3. Our backups include uploads and they are at 8GB now. The RO time during the backups has stayed roughly the same (actually since out last CPU and RAM injection it’s gone down).
Keeping much more than 2-7 backups probably isn’t going to help you much. If you get a massive spam/troll attack and don’t notice it soon enough then going back more than a few days will mean you’ll end up throwing away good posts while trying to weed out the bad.
Our backups were about 3.4 GB when we finished the import from our old platform, so the size of the backups are growing at a rate which one might consider alarming. We may end up shifting uploads to S3 and excluding them from backups at some point, but for the moment we’re just keeping an eye on the situation.