I put a couple drops onto my nipple before a feed every 4 or 5 hours through the day. Didn't worry at night.
It's better this way coz if DD has thrush in her mouth then it will clear her up too. It is supposed to be held in the mouth for a bit before swallowed, but babies can't really do this, so if it is on the nipple it doesn't get swallowed straight away.
Hope it helps.