I plan on starting to spread next month and will continue until I put the girls over in around May or June. I don't think you have to keep them out for any amount of time. I only have about 7 acres of pasture but they are over there all summer so poop does build up but I don't pick it up, just run a harrow to break it up and keep the flys down. Our soil is always nitrogen poor so I figure the more manure I can put down the better. Plus I put down regular fertilizer a month or so before I turn them out.
"Until one has loved an animal, part of one's soul remains unawakened..."
- Anatole France