Generally speaking for turkeys you butcher Great White and BB bronze at around 20 Weeks. Most other Heritage type you butcher at 30 weeks. This is plus or minus about 2 weeks. Generally 30% of the live weight is lost during the initial processng.
This is usually based on the best feed to weight ratio. in this range they usually have the optimum natural moisture content. But these are simple guide lines. Depending on your resting, and pre cooking methods you can keep them for a year or more and still have a moist tender turkey.
For the most part most growth slows down at around 9 to 10 months, So longer then this is just wasting feed.