Hi Paula,
I think you will find no specific guide for your exact use case. It is not a typical situation except for hosting companies (like Moodle Partners) or some larger institutions, but it is not a very unusual one either. But it is certainly more about how to generically manage multiple php installs most effectively.
As linked in this thread, and others there are quite a few support pages for general usage of GIT in Moodle Docs and elsewhere like linked in this other thread -> http://moodle.org/mod/forum/discuss.php?d=183693
Generally speaking you seem to be needing
When to update?
Well, technically there is a schedule and notification method.
a) make sure you have registered your moodle sites so that you get the update / security notification emails.
This way you will at least get an email about the changes.
b) You should schedule to come looking every 2 months at the most. The past release schedule is here -> http://docs.moodle.org/dev/Releases and the roadmap is here ->http://docs.moodle.org/dev/Roadmap
Expect minor releases every 2 months, and a major one every 6 months (Dec/June). So plan for these.
However, when to upgrade will probably be this timeline incorporated into the training schedule for your staff and so on. The major releases will have functional changes and improvements that may require training to be planned, scheduled etc.
How to update?
Although there are many many ways to do so, quick question - have you done an upgrade before? Have you used Git before?
You may want to have your development server separate to the production server, and manage the code there and push it into each of the different instances using different branches.
You may want to look at capistrano - https://github.com/capistrano/capistrano/wiki/Documentation-v2.x which has a good following. They have some excellent documentation.
I am no expert on GIT, but there are quite a few such gurus in the forums here. If you run into problems following the existing documentation, drop another question in here with as much detail as possible and someone is bound to know!
Hope that is of help.