As above, a bitch comes into season all her life. If you are not going to breed from her I would consider having her spayed. My bitch had a pyometra (an infection of the womb) and had to be spayed aged 8 and it took her a longer time to get over the operation.
An operation on an older dog (like humans) is harder to get over and is more serious. I had younger dogs spayed and they were over it and back to normal in a day or so.