Ask HN: Expected size of paying customer base with a freemium model?

I just finished listening to an interview with Drew Houston, co-founder of Dropbox, on the podcast 'How I built this' (episode published on the 9th of November 2020 for those interested).

During the interview it was mentioned that as Dropbox operates using a freemium model only around 3-5% of the total userbase are paying customers.

This got me wondering, is this the expected percentage of paying customers when using a freemium model (when successful)? Or does it matter if the product has built-in inherent virality to it (is the number usually lower than 3-5% and Dropbox has it better than most)?