How early do we need to be at BCN for flight to Granada

First, you should determine how you are getting to BCN. I would recommend a taxi that early. You should treat the flight like any domestic flight in the US.

I'd plan to be at the check in desk by 6:30. Not at the airport, at the check-in desk.

6.30 could be a little tight. I have seen check-in desks closing 45 mins prior to the flight. BUT --- am not familiar with BCN. Personally I would go for 6 am.