As usual, it depends.
If it's a job that I'm not bothered about losing, I will simply say to contact me when they're ready and I'll see if I have any gaps. I don't hear from them again. Sometimes I simply don't bother going back.
If it's a job I really want to keep, I tell them that from the spring, I will start charging at about one-third higher price as I consider that it's a similar level of cleaning to my 12-weekly customers.