“Permanent” in the sense that you won’t quickly lose it. But I’m beginning to suspect that if you quit PE - as in for many years - you’ll tend to start losing your size. I have no evidence to support this contention, other than the general observation that penis size does tend to shrink with age (this is even stated by the medical community).
So, if you PE your way from 6” to 8”, by age 25-30 say, I don’t believe you’ll still have that same 8” 10 years after you’ve completely quit. Especially when you start getting into your 40’s, 50’s, etc.
However, I suspect that only a modest maintenance routine (10-15 minutes every other day) would help you retain your hard-earned gains for decades to come - and it’s generally healthy for the penis to do some mild PE after you reach your size goals.