Depends. If you have a lot of users posting a lot of pictures and you use pictrs out of the box config, then a lot. If you are just running a few users with finite communities being synced then a lot less. The number is going to vary a lot as lemmy grows and gets older so hard to document realistic expectations. But docker images are probably going to take up more disk space than actual contents unless you get quite big. I just threw my PG volume into a tgz to move servers and it’s less than a gig.