You don’t lose gains that often. And a lot of the time, even when you’ve plateaued, continuing your routine will cement what you do have even if you aren’t gaining. Big losses are usually from newbies who had their newbie gains of like 1” and then dropped PE after hitting the goal like a hot potato the first day they measured a full inch bigger then they wonder where like .5” of it went in 4 months.
If you stay diligent, I want to say 6 months or so but don’t quote me, from the last time you measured then you’ll maintain that size. Generally. Keep in mind your routine may be lightweight, heavyweight and I don’t know how well or easily you’ll cement your gains as I don’t know your penis but…in my experience I haven’t lost more than where I was 6 months prior when I have lost gains.