Girth work takes time, and for some it can take couple of months while for other can take couple of years, this question cannot be answered very precisely.
As far as if gains are permanent, once you reach your targeted goal, it is most likely that you will loose some of that gains overtime. So advice would be once you reach your goal, continue with your routine (or slightly lighter if you do not want any additional gains) for few more months at least, as longer the better…
However, if you are serious with PE consider you will have to dedicate and “sacrifice” at least one year of your time to do the job done, properly.