I guess it would depend on size of paddock but we found that 6 mths was just about ok to grow the grass again over late summer and autumn for use in winter. It was soon demolished with all the rain that we had, the ground was so soft and heavy. Our sows trashed a new grass area in a fortnight
Obviously it depends on the size of the paddock but we try to limit pig usage of a paddock to 6 mths.
You might want to check out tuckers seeds or something similar and have a think about growing fodder beet, kale or mangels etc , rather than grass....it can be used as feed
but you would need to find ways of having your pigs graze sections rather than all at once.