The time depends on the breed & the weight your trying to achieve.
My experience with La Bresse & LS is 24-26 weeks would get you around a 1.5 - 2kg bird dressed.
But the trick is working out the ideal age & weight so you get a meaty carcass and not just frame.
I feed mine very simply, chick crumb for 6 weeks then growers until there processed, I also give them some scratch mixed corn twice a day and free range them., only containing them the last few weeks.
The French finish the La Bresse in small containment feeding bread & milk hoping for a tender pale meat but I've never tried it.
I'm doing 2 types of hybrid table bird this year a Slow growth Sasso Galouise which should reach 2.2kg live in 14 weeks and a Medium growth Sasso Farm Ranger which made 4.5kg live weight last year.
The problem with the meat hybrids is you have to be careful not to over feed them as they will put to much weight on and damage their legs, that is why I go for a slower growth bird, which still gives better results than a traditional breed.
I have a project going where I'm looking to breed the two hybrids together and blend in some traditional breeds for strength of carcass, which hopefully should give me a sustainable meat bird I can breed from.