I always take one shower a day when I'm going to be around other people, especially if I'm engaged in something like volunteering, work, or school. Sometimes I take it at night, sometimes in the morning... depending on how early I think I'll be able to get up.
However, if I'm just sitting in the house for several weeks alone, I tend to take one shower every three days. Mostly because I become apathetic when I have no one around to care whether I'm clean or not. Same goes for cleaning the house. If I know I'll be having company, I start trying to put everything away and make everything look presentable. But if I'm alone, I let clutter pile up.
Incidentally, this can be a self-reinforcing cycle, because if someone wants to come over on the spur of the moment, I'll panic and try to stop them from coming or even refuse to answer the door. Because the place is a mess, and I probably haven't showered in two days. I won't tell people that's why I don't want them to come over, though.