I think that probably depends pretty heavily on what your app is. One of my clients has about 200,000 user accounts (and 40K actives/week) in a database that's well under 4GB, and it's been live for three years now. It's in Postgres, but there's nothing that'd stop them from using a 10GB-capped SQL Server Express until a point where having to pay for SQL Server is a good-problem-to-have.