Ive seen 3/8" permanent girth gains in about a year. The first 1/8 I don't count too much as I feel I was getting back to what I used to be years ago. After that it's all Bathmate and all new.
I used it as advertised for the first six months and all I got was the first 1/8"
After that I started using it on lower pressure for longer sessions. 30 minutes, an hour, 1 1/2 hours. Many times I'll take it off at 30 minutes , get a new erection, get new hot water and start again.
I've seen huge results becuase of this. I have two friends that also have done the same thing and are getting similar results.
I really think the low pressure is the answer. You don't want to harm your dick just get it to be used to being big. I think more guys overtrain then anything. Under training leads to slow gains overtraining leads to no gains.