I used to milk some after the foal had gotten theirs, usually about 8-10 hrs out. I'd normally use a breast pump and took it over the course of the next 8-10 hrs, a couple ounces at a time to not deprive my own foal. I put it into heavy freezer bags so that I could take almost all of the air out. and wrapped to protect from light. My preference was to keep it in about 2 oz portions as I could thaw several or only a couple, not to waste it. I would also extract some amount of milk over the next 3 days in a similar fashion, mark mare name, date, etc. on each pkg. In case I needed to help a foal and it was a time when I couldn't get to a place that sold milk......and/or to mix with it to allow a transition.
When I was breeding 20-30 mares a yr, I always had a supply. Vet said it would last 1-1.5 yrs if kept frozen, prepared well, etc. At the time I also had a herd of heavy milking goats who were being freshened frequently for kids to sell (meat goats & nubies).....so, there was a spare freezer in the garage. Some of the goat milk was used by a wildlife rehab guy for fawns. But, it did give me a nice supply of rich milk if needed for a foal. Fortunately, only ever needed it one time for about 2 days. Shared with others on occassion. Vet would call me if he needed such.
OH -- important that when you thaw it is NOT microwaved or heated in a pan. Lukewarm water in a bowl and put the container of milk in there to thaw. Change water to keep it thawing. The smaller sized portions will thaw easier, IMO.