We keep two weaners for about 6 months, April to October, on 2 pens 30m x 40m roughly. At the end of the period, the pens are mostly mud, but that doesn't seem to bother the pigs. Although we're close to the river, our land is fairly free draining and by the time April comes round again, the vegetation is regrown ready for the next two. If we had pigs all year, I wouldn't keep them on this land but with a 6 month break, it seems to do fine.
The original plan was to have three pens, one for pigs, one for veggies and one growing a forage crop for the enxt batch of pigs but it didn't quite work out like that.
Rosemary