I seperated my babys for 4 months. And now they are together with their moms again. It can take 6 months to a year. If after you put them back together they start trying to suck you can seperate them again but I doubt mom will let her. Keep an eye on mom for her to dry up. It helps if you have another horse to put the baby in with but if you don't be strong and keep them seperated. Some say a mother will wean the baby herself after time but that isn't always true. Its sad to see a 1-2 year old still sucking on her mom
"Until one has loved an animal, part of one's soul remains unawakened..."
- Anatole France
I was going to say at least a couple months. Best if you can put baby with another baby. If not, I've had mom and baby share a fence but it must be safe enough that the baby can't stick her head through, or even try, to nurse.
Even though the moms appear dried up, and they may be, they can start nursing their babies again if your too quick. I learned that one the hard way.