Hi younggun...don`t think I`m being `smart`with you..your head doesn`t have a perimeter, perimeter is the system, or more precisely, the edge of the system where it meets your growing hair. Judging by what that chap said, you could expect to get about 3 months, providing you use good conditioners etc., as you intend to expose yours to a lot of salt water etc.... mine is now going into it`s 6th month and it`s practically as good as on day one, but I`m very easy on it as you know...you won`t be easy on yours, but still, if you get 3 months for that price, I`d be happy with that ...means you`ll need approx. 4 systems a year..... many people pay 2 thousand dollars from ``other places`` for just one !!! Regards, Paul.