I would love to hear the answer to this as well. I guess it’s heavily dependent on how many users you have and if they are uploading images or just text, or just lurking.
I think if you did not allow signups it would probably be extremely minimal.
Mines been running for a few days now and with some image heavy communities shortage is up to 1gb for 1 user. I have it on a separate volume that can be expanded if needed.
I assume an instance with users subscribed to active communities requires meaningful storage, but I'm not clear the sizes we're talking about (what's data growth per/day been like for some of the larger communities).
EDIT: Likewise, I'd love to know if that 150GB RAM is fixed or whether that number grows with use.
I suspect that it depends on if you're accepting other users and or communities. If so, you'll have to be global mod and probably spend time sorting out mod reports etc.
If you're just setting up an instance for yourself, it's probably easier but you'll need an account somewhere else to discover communities and index them back to your instance.
Lemmy starts out empty, and if nobody joins any communities it doesn't start getting any data. It doesn't bring over old data, so if you install it on July 1, the data just grows from July 1 onward.
Lemmy is going through some growing pains, performance wide and federation data sharing wise. Hopefully 0.18 will be out within a week or so and and some of the worse performance issues improve.