If you talk to a vet and read all the books on weaning the recommendation is usually 4-6 months. It depends on several things. Is your mare is holding her weight and is she being kind to the baby? When it starts to get where a mare is dropping weight even though she is getting the right nutrition or her teats are getting sore from the babies teeth, or a mare will even start to kick at her baby to get it to stop nursing then it is time to wean. when a babie is spending more time playing then with mom or eating other things all the time and only comes back now and then to ' play nurse' or out of 'habit' then it is also time to wean.