You are hearing a lot of different answers because so many vets do it differently.
I think it's fair to say the most common time frame is when the kittens are between 8 and 12 weeks. Some vets will do it earlier if necessary, some prefer to wait until she has completely weaned the kittens.
So, ask the vet who will be preforming the surgery at what age they prefer to do it at. Also, keep in mind she may come into heat before then. While the surgery can be performed during a cycle the risks do increase and some vets won't do it at all. Go ahead and make the appointment early so you can get on the schedule but know you may need to change it if she's calling during that time.
Until the surgery happens she must stay indoors and away from any intact males. Given that she was already pregnant when you got her you don't yet know what a heat cycle is like for her. Cats do not bleed like humans and dogs do but many let you know by becoming quite vocal and shall we say, quite amorous. A lot of females will "present" themselves to anyone and anything (butt in the air with the tail low and to the side). Others, however, are silent callers. They show absolutely no signs of being ready to mate. Since you don't know her signals you just have to err on the side of caution and keep her on total lockdown.
Also bear in mind that cats become amazing escape artists when they want to go meet up for a tryst and Toms will come from miles away to try and be the lucky suitor. I had a Tom scale my second story balcony and almost knocked down my screen door trying to get to a Queen inside. So, keep the windows and doors firmly secure!
So, ask the vet who will be preforming the surgery at what age they prefer to do it at. Also, keep in mind she may come into heat before then. While the surgery can be performed during a cycle the risks do increase and some vets won't do it at all. Go ahead and make the appointment early so you can get on the schedule but know you may need to change it if she's calling during that time.
Until the surgery happens she must stay indoors and away from any intact males. Given that she was already pregnant when you got her you don't yet know what a heat cycle is like for her. Cats do not bleed like humans and dogs do but many let you know by becoming quite vocal and shall we say, quite amorous. A lot of females will "present" themselves to anyone and anything (butt in the air with the tail low and to the side). Others, however, are silent callers. They show absolutely no signs of being ready to mate. Since you don't know her signals you just have to err on the side of caution and keep her on total lockdown.
Also bear in mind that cats become amazing escape artists when they want to go meet up for a tryst and Toms will come from miles away to try and be the lucky suitor. I had a Tom scale my second story balcony and almost knocked down my screen door trying to get to a Queen inside. So, keep the windows and doors firmly secure!