Well if you use it for 2 weeks it will grant you temporary gains, which will go if you stop.
However, you said over a 'couple if years'. Say you mean 3 years, and each year you gained 0.5 inches. By the end of the third year, i'd say the first 2 years gains, 1 inch in total, will be 100% cemented. The rest may vary from person to person, but i'd expect to see atleast 0.2 inches of the 0.5 inches gained in the final year to be permanent.