Mine has always gone into heat after weaning EXCEPT for this year! They were coming back into heat about a month after kidding. I have no idea why they decided to do this annoying little change. So play it safe and keep the boys away until your ready to breed
I think it can really vary. If you don't have a male around them after kidding then I think sometimes it's not as noticeable? I will say though I notice most of ours show no real signs of heat from spring until about the end of July, and since we normally kid Jan-Mar we don't see heat in our mature does, and only randomly in the young unbred girls.