Commercial Landrace are typically put to the boar at around 8 months old, so have farrowed by the time they're a year old and they produce around 3 litters before being culled, I assume because they've been pushed so hard having been put back in pig a month after farrowing, that they're clapped out. I would expect a native breed sow to farrow at 15 to 16 months and produce at least 10 litters (being put back to the boar after weaning at 8 weeks) before her milk production could no longer keep up with demand from her piglets.