Ask HN: How much space do users actually use?

We have a new photo SaaS and I'm working on some cost projections.

I know all apps and industries are different AND I know people don't want to give out numbers that would reveal revenue, but I'm wondering if anyone can provide some basic guidelines on what to expect in terms of usage ...

When you have service with files and storage quotas, what % of that quota might users consume (on average)?

I know cheap web hosts oversell capacity. Is there a rule of thumb they use? Can you do the same with S3 storage/bandwidth?

TIA