you should be expressing...when they get very hard. Take the milk off the top, enough to relieve pressure but not enough to stimulate. Otherwise you could get mastitis...
and trust me, from someone who had severe mastitis (worst case any of the 3 midwives and breastfeeding support worker had ever seen in their careers!) that you REALLY dont want to get it...the milk ducts become so blocked and it can lead to abcess, you will feel like you're coming down with flu and your boobs will be really hot and red....
seriously not nice, trust me. Even a hand pump would do!