Yes. What you gained will stay permanently. Like everything in your body, there are three stages: growth, sustain, and regression.
In PE, you also have 3 stages: growth/gain, sustain, and regression.
The growth/gain stages is temporary until you cement it. It's part of growth phases. Cementing request just a bit more time, roughly around a month to 4 months after the growth. It's to prevent any possible immediate regression or loss. It's just like your weight lifting gains. You build muscles, you don't cememt the temporary gains with cementing work, you'll lose the muscles within a month or two. Once you cement it, the sustaining PE workout is very light. It's as light as doing one routine every month or so. It's just to prevent the tissues from regressing due to aging. Regression is just that, the regressing due to aging. Just like the rest of our body, every part of the body will shrink or regress due to aging. Ever heard the phrase, "You don't use it, you'll lose it"? Just like your penis, you don't maintain the blood flow, the light exercsises and the prevention care, your penis will regress. The loss is very little compared to what you've gain.
Yes, after you cement your gains. You can completely forget it you like. But remember, age is not just a number. It's an indicator of what you will lose as you grow older.
Yes, you'll keep what you've gained after the cementing work.
Only if you want your 100% gains to remain, or even adding a bit extra.
If you're satisfied with what you have, there's no need to pursue any further. Once again, the aging regression.
If you're only chasing after 2in length and 1in girth, then it doesn't take 5 years. 5 years journey in disciplined PE will allow you to gain as much as 5in length and 4in girth. Most of us gains around 1in length and 1in girth at minimum each year with disciplined PE. That means you can't be relaxed and lazy about the routines. Otherwise, it will take you 5 years just to gain 1in length and 1in girth being relaxed and lazy. You'll be surprised that doing very light maintenance PE will actually make you gain at least 0.15in length and 0.1in girth yearly.