Depends on your opponent, his/her actions in THIS hand, their chip stack, and their perception of you. Sorry, it's a bit vague but it is complicated.
Example on the river, i believe my opponent has either a pair of aces or two pairs. I'm holding the nuts (nut flush). She bet the ace on the flop and turn but checked the river which gave me the flush. She's tight. Bet 1/3 pot as i am not going to get more here.
Same example against a looser player, I would put in a little bit more than the minimum raise, looking for them to suspect that I do not have the flush and to push. Then I re-raise.