I would say as soon as you are sure your mare is cycling. None of mine are at the moment - I think the weather is confusing them. I have one who was doing lots of winking last week but then didn't go any further, another who came in season for a day a few weeks ago and we've seen nothing since and one who hides her seasons anyway so I'm not sure!
Is yours on regumate for pain/issues or keep her regular? You could ask your vet to scan to see if there is anything going on?