The gains are as "permanent" as muscular gains made in the gym. If you don't continue to workout those muscles, then will deteriorate, and get smaller.
Bathmate works, but you have to stick with it. Eventually, the temporary gains found immediately after use will start to match your normal size. I'd say less than a year to expect those results. In fact, most guys start to see "permanent gain" around the 6 month point.
You have to be realistic about this stuff. Great gains are possible, and in a safe procedure. Just don't set yourself up for disappointment in the beginning! You probably are NOT GOIN to gain an inch in a month - but some people have done so!
Approach this with the same mentality as weight loss, or muscle gain in the gym. Stick with it. Allow your body to rest for an appropriate length of time, too. Gains are made in recovery, not constant exertion.
Try it!