I held down a letter key for about 30 seconds. I hit save. Then I looked at a public view of the profile. The other last name field continued until the edge of the screen outside of the profile page. So I guess the answer to your question will depend on the size of your monitor. Update, Upon shrinking the screen I find that it only carries on about twice the length of a profile page, probably about 200 characters, but well beyond the range of view. It could carry on indefinitely like a spreadsheet but it would take a great deal of careful use of directional keys to read it.