You can start at anytime. Some mums like to give their babies a top up feed just before they go to bed while baby is still asleep (saves waiting for them to wake or being woken just after falling asleep yourself).

It never made a difference to my babies, they would wake at their usual time anyway. It really only worked if it was actually their feed time and they hadn't woken, but it can backfire and wake up baby instead.

I found following baby's cues easier. It's a long road to surrender this motherhood business. It's worth trying a dream feed if you like, it's worth trying everything I think, because every baby is so different!!