In my limited experience, even the heritage birds require different amounts of time to fill out. The standard bronzes seem to fill out faster than the bourbon reds, which are a little faster than the narragansetts, which are a little faster than the royal palms. At least with the lines I have, I think I could butcher the standard bronzes at 25 weeks, while 30 weeks would be better for the Narragansetts or Palms.
Ordering heritage poults for mid April will get you about 30 weeks before T-day. Broad-breasted for early August will get you 15 weeks.