I am with themacpack. I wouldn't even be contemplating riding your mare. By the time you have her weight up to where it should be, it will more than likely be weaning time for that foal... So I don't see why you are asking... unless you are actually wanting to ride her now with her being underweight.... I know you are riding your "stud" while he is underweight, so that wouldn't honestly surprise me.
As to when others do it... well that varies from horse to horse and situation to situation. There is no set standard.