How common is it to hire someone to clean your house or apartment?

There are a couple families in my, mostly middle class, apartment complex, that hire a cleaning service to come every couple of weeks. I also know people that have someone come over to clean their house once or twice a month. How common is this?

Edit: wow, I thought that the answer would be that it is somewhat unusual, but it seems from these comments that it's way more common than I thought. Also, it seems like every two weeks is the norm for some reason and I don't know who decided that custom.