If it's a CV, it's not important at all. Write a book.

If it's an American style resume, then it's pretty important but not superlatively important. Your resume can be 2 pages (maybe 3 if you're awesome), but assume that the second page will never get looked at (and make sure your good stuff is on the first).

Edit: Also, it helps to have multiple resumes. My project management experience is ~2 lines on my programmer resume, but on my project management resume it's most of the page (obviously).