Seasons vary from bitch to bitch, and although three weeks is the norm, they can last longer. I had a bitch mated on day 28 after the males had been in kennels for three weeks! Once she has gone out of season its usual to spay them three months after, as this gives time for the system to return to normal and hormone levels to settle.
It also ensures she is not actually pregnant as some people might not be aware and might take her to be spayed whilst she was carrying puppies.