Dear Bob:
Regular lessons are useless after some years. You only need them until you can move alone. Of course, some kind of orientation is welcome then, but less frequently.
It changes a lot, but I would say that weekly only works to amateur who play only for fun. In university, it would be much more interesting to have a couple of hours monthly, although I'm talking something that is hard to achieve, because I'm a professor and I have to give classes every week to my undergraduate students.
If you have contact often with your teacher, s/he will solve a lot of things that you might be able to solve by yourself (fingering, expression, ways to develop technique and to learn). When you have to do it by yourself, you have the opportunity to find your own path, and that is of paramount importance in the shape of a performer. When you reach something about a semi-pro level (i.e., you play recitals and the like), you probably need just three or four meetings a year. And if you have this classes with different teacher, it's even better.
Best wishes!