one week

For the full JR Pass, you MUST buy the exchange order for it before you arrive in Japan. You cannot buy it in Japan after you arrive.
But whether it is worth the cost or not depends on where you plan to travel. If you do a lot of long distance travel, especially by bullet train, it can save you a lot of money. You can check regular fares on This is HyperDia and see if the total exceeds the cost of the Pass.

If not, there are various regional passes that you might find helpful. See:

http://www.japan-guide.com/e/e2357.html