Generally anytime from March-October starting when they're 4-6 months old.
You should spay her before her first heat to reduce her chances of
developing mammary cancer later in life.

It really depends on each individual cat. I have seen some females go
into their first heat as early as 5 months of age, and others not until
they are over a year old. Usually a female cat will go into heat at
around 6 - 8 months of age. I am hoping that you get her fixed at the
earliest possible opportunity. The last thing this world needs is any
more little kittens running around with out proper homes for them. You
can contact your local vet to find out when the best time to get her
spayed would be. Once you know, make the appointment and then go when it
is time to do so.