Are you sure she has not already been done? If not and you know when her last season was then you would usually spay them about three months after the last one, i.e. about halfway between seasons which are usually (but not always) about every six months. Sometimes if they are in poor health or condition they do not come into season, as nature would not allow them to have puppies in that state. I would book her in at your vets for an examination and ask for her to be booked in to be spayed, They will probably do it anyway, unless she comes in before the date booked.