When we had Miss Cerridwen Blythe (12/1983 - 04/2000) spayed, the vet told us it would take a month for all the hormones from her "heat" to clear out.
We rescued her during a very cold January week in 1987. She went into heat a few days before the appointment we had to have her spayed. She had had an initial checkup when we took her in, before introducing her to our other kitties. She was such a sweet cat that they took to her right away.