I expressed for donation last time. Was five years ago so dont know if its changed much but they would take whatever you could get to them. At first I was getting about 4oz a day but got going til I was getting 10-15oz day from three/four sessions (this was when my son started dropping milk feeds in favour of solids so I picked up his milk feeds with expressing) When I lost a baby but produced milk I was getting about 3oz a day, this was somewhat more difficult but kept it up for a few months.
Some milkbanks collect but others dont, you would need to find out about your local one. They do provide all the containers and labels which is useful. Most of them will collect every few weeks so if you manage a fair bit you need to clear an entire drawer for storage...I made the mistake once of not having enough freezer space and had to store milk in my mothers freezer, she wasnt that impressed!! My milk bank didnt collect from me, I took the milk to them as it was quite a distance, a lot of people find distance from a milkbank to be the restraining factor on whether they can actually donate or not.
I noted that your LO is over 4 months, they do need to be under 6 months when you start to donate but you can donate for as long as you are producing milk if you start by this point.