I think it depends on what's being discussed. I'd say 'a few' days would be 5 or 6. If it just meant three, there wouldn't be a need for such a phrase!
Somebody told me today that a relative was staying with her for a few days. I thought this meant about 3 days, but she says a few is about 7! I would have called that several! The dictionary says a few is not many, a good few is several, and several is a few!!!! What would you say?