Hi,
A lot depends on whether you want them for pork or bacon. Obviously if you want bacon then you grow them on to quite a big size (over 75kgs dead weight) but ours generally go for pork and our butcher likes a dead weight between 55-65kgs. Our pure pedigree old spots normally reach this weight around 26 weeks of age, but your question obviously depends on the breed of pig and what type of feed you give them (i.e the protein levels etc)