I think the short answer is when you have time. With an unpredictable baby sometimes you just have to fit in when you can. Having said that, many mums find that they get the most milk after the first morning feed, or during the night (although I for one never had the energy during the night). Some mums who are more co-ordinated than me manage to express from one side while feeding from the other. If you can manage this it can work really well because the feeding baby will help you let down on the other side for the pump.

Another thing that can work is expressing straight after a feed starting from the second side, or even half an hour after a feed. You don't have to worry too much about your little one wanting a feed again soon after expressing as your supply will build up again quite quickly - often within 20 - 30 minutes - so there will be milk there when it's needed. In fact, you are never totally empty, there is always something there.