The first line of the commit message should be 50 characters or less. The rest of the message goes on the second line but wrapped at 72 characters.

For reference: http://git-scm.com/book/ch5-2.html

Example:
Short (50 chars or less) summary of changes

More detailed explanatory text, if necessary. Wrap it to about 72
characters or so. In some contexts, the first line is treated as the
subject of an email and the rest of the text as the body. The blank
line separating the summary from the body is critical (unless you omit
the body entirely); tools like rebase can get confused if you run the
two together.

Further paragraphs come after blank lines.

• Bullet points are okay, too

• Typically a hyphen or asterisk is used for the bullet, preceded by a
  single space, with blank lines in between, but conventions vary here