The 50/72 model seems to be a good practice. i.e. ... the first line should be maximum 50 chars long and should server as a header. Followed by a space, the second set of line(s) should be wrapped at 72 chars and should serve as a summary. Here is a SO question : Git Commit Messages : 50/72 Formatting, that discusses the same.
Here are some exhaustive notes on the subject:
- GIT Commit Good Practice
- A Note About Git Commit Messages
- Proper Git Commit Messages and an Elegant Git History