How would you expect a revisions system to work?

I need to keep a revision system of changes. My question isn't the structure but the behavior.

For example, let's say you are working on a webpage in a code editor. How often would you make a revision for the developer as he's coding the page? Do you save when he saves? Do you save every hour? How many revisions should you keep?

What behavior would you expect in this scenario?

Update: It seems that it makes a difference if the content is saved online versus saved locally. In my project it's saved online like, WordPress posts or Google docs. I can save locally to the cache or even to a directory on the users computer if they use the desktop software.

Solution

When you are versioning, you:

make sure that teammates will not override each-other's work
write a version diary for future use to be able to handle any scenario

Since it is possible that at some remote time in the future you will need to roll back, it is better to make sure that upon looking at the commit messages, you will be able to determine easier where to roll back. Also, commit messages are helping your teammates to determine what did you do. As a result, my advice is that you should make a commit whenever you have completed a small part of your work, so your commits should be atomic and their message should be speaking for themselves.