I went back when Ellie just before Ellie turned 5months. Since she was 3months I had been reguarly expressing milk after her morning feed and freezing down a supply of milk in small bags. To start with I was only getting off 1 or 2 oz. This increased until I was getting off anything upto 5oz (it varied probably depending on how much she took for her breakfast).
I timed my day so that I gave her her morning feed at 7:30 and dropped her at nursert at 8am. I arranged shorter hours at work so that I could pick her up by 4pm and feed her the instant I got through the door. This meant she only needed 2 milk feed during the day. To start with she only took the bare minimum from the bottle and made up for it when we were together. I expressed around the same time as I would normally feed her. I used to find I got loads off at the 10:30 feed but hardly any off at the 13:30 feed. Most of the time that was enough but when I came down ill I couldn't get enough off and was raiding my frozen stash.
I also started weaning early, (at 4 months) to try and decrease the amount of milk she needed. As weaning progressed she took less and less during the day. She still has 1 bottle during her time at nursery however, often she only takes 2oz and sometimes she refuses it altogether. Once she hits 1yr I will try and get her to just have a glass of cows milk then rather than expressed milk.
Looking at your LO's age I expect you will start weaning soon if you haven't already, so that will help.
Sorry for the long post. I hope this helps. The expressing feeding support thread might help a bit as well.