It depends on the vet doing the spay. Some will do pediatric spay/neuter, others wait until about 6 months. There are benefits and drawbacks of each. My female was spayed at maybe 4 months (somewhere int he 12-16 week range); last male baby had to wait until 5 or 6 months, though he hit the four pound mark at about four months (he's huge). Different vets, different ideas. Both cats did fine with the procedures. Personally, I like to have it done as soon as the vet feels it's safe.
I have a tiny 6 month old kitten here who weighed in at just over 2# at about 4 months old. My vet told me he likes them to be about 4# before he spays. Due to the weight restriction, she's gone through 2 heat cycles and has her appt for the 14th of this month. It's going to be close as she was 3.2# just after T'giving. If my vet wants to wait another month, he gets to live with her until he is comfortable doing it!
With cats, every vet around here knows that the safest with healthy kittens is to spay them as soon as possible, because the very real chance of pregnancy and complications from it is more than complications from a spay because she was not quite grown yet.
Plus the world doesn't need one more litter of cats.
Ours have always been spayed on a guess, didn't know how old they may have been, just got it done asap.
Is she a 100% indoor cat? If so the drawbacks of pediatric spaying may outweigh the benefits. If she can ever get access to the outside, then call and ask thevets weight minimum. At our closest shelter it is 3 lbs for owned cats and the vet may do smaller feral cats on a case by case basis. Similar rule at the vet clinic I was with but he preferred to wait until they were bigger if the owners were responsible at keeping them indoors.
The vet hospital I worked at wouldn't do them if they were less than 5# and at least 6 months. When I had a litter of kittens that I adopted out privately, I had them all spayed and neutered at 10 weeks- they were between 2.5 and 3#. MUCH faster recovery and I knew for certain they would not be reproducing.
The Alabama Spay Neuter place here will spay as soon as they are 2#. I was kind of happy about that as I had three babies that I needed asap as they were going to be outdoor cats. (They are currently on my bed. ) Many regular vets here require them to be six months old.