So, I've been running my personal instance of ONI for abut 2 years. I follow about 34 other fediverse accounts (only two on mastodon.social).
It has stored over this time about 3.2GB of data spread over 345K JSON-LD documents (mastodon.social is responsible for about two thirds of the size)
However the thing that's surprising for me the most is that despite the fact that I'm using the least "web scale" storage available - plain files on disk - the instance is still very responsive. The server also does a lot with collection filtering in order to display a meaningful timeline to me, so it's not just a dumb pipe from disk to browser.
