The mare will spean the foal herself if you just leave them together. If you haven't covered the mare again, there is no real need to part them.
I have two home-bred horses here. They both still run with their dam. The elder, a 4yr old gelding, stopped sooking his mother at about 9 months old. The younger, a 2yr old filly, was still having a suck till she was about 20 months old. She's stopped now. I only cover my mare every two years anyway, and she's empty this time, so it doesn't matter. All mine live out as a herd, so the closer they are the better.