The traditional rule is : once per month for the first year, and then yearly until high school graduation.
This seemed like a lot when I first entered the field, but I soon realized the emotions behind the tradition. During the first year of life babies change so much from month to month. After I had been photographing the same children for an entire year and I brought up the images side by side and was blown away by the evolution of a child through that first year.
There is just something about a professional photograph that really captures the essence of the moment. With the first year you don't need to buy many prints each month, but it is a priceless record of your child's life. Some photographers even offer packages that will include monthly sessions for the first year to assist in the process. It is one of those things that takes time, and money to accomplish, but also something you will not regret.
The image in this post is ©2012 Brooke Photo Studio (that's me!).