I started to express when LO was 2 weeks old so daddy could do the 1am feed and I could get a few hours sleep. By 6 weeks he was needing about 6oz and it was a pressure to make sure I had expressed it so we started to give him formula for that feed. I carried on BFing the rest of the time and expressing for when we went out. I would express 3 times a day to get a 7oz bottle.
My LO would happily swap between breast, bottle and formula and we gradually increased the formula so he was 50% breast and 50% bottle. This gave me more flexibility and daddy could also take charge of feeding and I could have a drink without worrying about how much expressed milk we had.
However, by 15 weeks my LO was refusing the breast except at night as a bottle is easier to get milk from and he was becoming lazy. I carried on expressing until last week when I've had to go on meds for a trapped nerve in my shoulder. At 18 weeks he is now totally formula fed.
I have really enjoyed combination feeding and would do it again if I have another!