depends what you want to do! If you intend to pootle round the field (with foaly following)at a walk then Id say almost immediately. If you want to get back into serious work Id wait till the foal is weaned. Some people seperate the foal from the mare for a few hours during the day but you would have to have the facilities to be sure that the foal would be confined and unable to hurt itself. To be honest riding a mare whos brain is focused on the whereabouts of her foal, seems like hard work to me and pointless.