It's a perpetual lease but there are actually a few legal restrictions.

http://www.nsgtmo.navy.mil/gazette/History_98-64/hischp1.htm

"In all provisions regarding the Guantanamo area, the original agreement (February 1903) and the supplementary agreement were later confirmed by the Treaty of 1934 between the United States and Cuba, signed at Washington on 29 May 1934. A copy is appended for reference. This treaty has the effect of giving the United States a perpetual lease on this reservation, capable of being voided only by our abandoning the area or by mutual agreement between the two countries. The so-called "Platt Amendment," which gave the United States the right to intervene in Cuba, died with this treaty. Bahia Honda was not mentioned, having long since been abandoned.

Thus it is clear that at Guantanamo Bay we have a Naval reservation which, for all practical purposes, is American territory. Under the foregoing agreements, the United States has for approximately fifty years exercised the essential elements of sovereignty over this territory, without actually owning it. Unless we abandon the area or agree to a modification of the terms of our occupancy, we can continue in the present status as long as we like. Persons on the reservation are amenable only to United States legislative enactments. There are a few restrictions on our freedom of action, but they present no serious problem. We may not use the reservation for other than a naval station; we have agreed not to interfere with the passage of vessels engaged in Cuban trade; private enterprise is forbidden on the reservation; and we are obligated to prevent the smuggling of materials and merchandise into Cuban territory."

Technically using Guantanamo to hold terrorists might be breaking a restriction on the lease. I suppose we can say it is a naval prison?