That's a good question. I've heard of pig farmers breeding pigs on their first heat which is usually is at 5-6 months of age. Once the heat cycle starts it'll last 8-36 hours. Sows will go into heat every ~21 days.
I personally wouldn't feel comfortable screwing a sow at 5-6 months of age. I would probably wait until they are about fully grown. Sows stop growing at 2-3 years of age. I'd probably wait until 1.5-2 years of age. But I'm a canine zoo and don't actually have any real world experience with shagging sows.