Hi
There is a commercial Turkey farm up the lane from us that rear 3000+ turkeys to supply butchers for the Christmas market.
Last year we purchased 15 day old turkey poults from them and they came in at the end of July. They were massive by Christmas. We bought a range of sizes from medium upto HUGE and the smallest we had was 16lb and the largest was 38lb. A mixture of Bronze and white, although I preferred the bronze.
So i would say that as long as you are getting a commercial double breasted strain then July is the optimum time. I guess you might need abit longer for the Heritage pure bred types.
Hope this helps. Hope you have fun rearing them, we absolutely loved them and am looking forward to rearing them again this year.
Best of luck
Steph x