I have read on here, people say 2oz per 5 gallons(or to taste). I think its a good idea with the bananas cause you don't get the juice like you do from berries. I would guess adding it to secondary or before bottling.
I made a porter with bananas. I blended them and let them soak in some dark rum for a couple on hours. Then racked to secondary. It ended up with a small hint of banana taste and some aroma. Next time I make it I am going to add artificial banana flavoring as well. But because I want more...