Hi tamworths are slow growers and in my experience not prone to putting on fat. If keeping for bacon hang on to her as long as you can, although best not to keep her after the others have been sent on, as she will be lonely. Tamworths produce wonderful hams and lean loins for great bacon.
As long as she looks filled out and you are happy with her weight, any time after 8 months old. Hang on to her for as long as you can, you wont regret the extra flavour.
My mouth is watering, tamworth bacon, ooooh bliss. HM
ps sorry should have said welcome to the forum.