The time varies greatly depending on the personalities of the mare and foal. If a foal has already started eating grain
(as from a creep feeder) and has free access to pasture and/or hay, It can take as little as two days. A lot depends on the mare's attitude. Some mares are a little tired of the whole thing and do not encourage the foal to nurse; while others will position themselves near the foal and guide the foal to the udder even lifting a leg to ease access.Typically, a foal of 3 to 6 months of age that is eating well and consuming quantities of water, when confronted by Udderware, will search and maybe bump the bag for a few minutes, get discouraged, and go back to grass or hay. This happens a few times and the interval between attempts, increases.
Eventually the foal just says "the heck with it" and quits trying. The owner must be observant of the process and respond to it. In any event, once weaning has started, allowing nursing again will stimulate milk production and you will have to start again with a foal that has now learned that persistence pays off. In one case, the process took 8 days.