[WalterBright]

If there are empty tables, it's fine to sit as long as you like. If there aren't empty tables, it's common courtesy to eat and go.

Seems straightforward to me.

[lovebug]

This is true, consider that having patrons sitting in your shop attracts others often enough that they will appreciate you being there if its empty. If it gets full though, buying stuff and leaving when you are finished is good form.