Hi JK, I had a situation very similar to yours with my ds1 except by 2 monyhs he refused breast completely I wound up exclusively expressing with an Ameda Purely Yours until he was 7 months old, then went to 1/2 ebm/ 1/2 formula. Here's what I did"
initially expressed every 3 hours round the clock for the first 6 - 8 weeks
then dropped to every 4 hours - still round the clock - until he was 6 months old
after that I went to 4 times a day while awake, none at night, and continued with this until the last month where I dropped a session once a week
I always averaged about 4 oz. per side, tpically more from my right than left.
Tips:
Stay well hydrated
Be consistent
Use breast compression while pumping!!!
Use a good quality double electric pump
Get the car power adapter with your pump - yep, I've even expressed in the back seat while my husband drove down the interstate
Some other good ideas here
One thing I wish I had tried before going to exclusive expressing is nipple shields. If you decide to give them a go work with a lactation consiltant first as there is a bit of a trick to using them successfully.
Good luck and HTH