The human body strives to maintain certain “basic” levels of strength and muscle mass.  It will always return to those levels if allowed to do so.  The maintenance levels are determined by requirement only.  Increases in strength and muscle mass can however, to a certain degree, be permanent.  Exercise is not the only way to maintain these increases and to build muscle fast.  Evidence shows that the body will permanently adjust its state of “normal” if changes are maintained for an extended enough period of time.

As an example. If you increased your level of strength by 100% in a period of one month, then ceased training completely, the previously gained increases in strength would be lost within a period of three to four months.  However, if you produced a 100% increase, and continued to perform enough exercise to
maintain that increase for one year. And, if then you ceased exercising completely, the loss in strength would be much, much slower.  Also, a portion of that increase would become permanent.

Without at least some exercise to maintain size however, even if you gain muscle fast, it will never become completely “permanent.”  The longer you maintain a strong and muscular body, the greater the permanence of your size and strength.  It’s not the rate of progress which determines longevity of muscle gains.  It’s actually the length of time those gains are maintained for.  The faster you produce your ideal muscular size, the easier it is to maintain it for the long term.

Also,  levels of strength and muscle mass which have been lost, due to lack of exercise, can be regained rapidly through the use of a proper program.  The length of time you have gone without exercise, however,  will affect the rate you gain muscle mass.  The longer the period you go without working out, the longer the time period to regain previously held levels of strength and muscle mass.