Do you think it would be possible to gain 3 inches with 6,000 hours of consistent use?
It might be, I don't know anyone who has managed that.
PeGym.com has a lot of data and info on using it and a representative from Phallosan also has a thread there.
The problem that many find is that their penis becomes adapted to the maximum tension so the gains slow down and high tension = higher risk of injury. You have to be very careful.
If you want to go for those 6000 hours, try 3 months of stretching then have a 3 month deconditioning break. From 6 months you can start stretching in the green zone again and possibly see more gains after you start wearing it. During the 3 month break you can try pumping instead which will target the girth more.
If you're ready to do consistent PE for several years or even decades, you might succeed in reaching your goal of 10". I've been doing PE on and off for 2 decades and still haven't gone beyond a 2 inches gain in length yet. I did waste a lot of time with devices and techniques that didn't work though, usually you should see if you are gaining after 3 months, if there is nothing make some changes to your set up or PE technique.
I advise you not to keep measuring too often, it can be discouraging because some days erections are weaker than others so your dick might be slightly smaller. Once or twice per month is enough for measuring, longer gaps make the small size gains more noticable which is encouraging.