Here's my honest, personal experience...I've been extending for 3 years now, BUT I'm not consistent with how often I do it. In total, I have done it for 816 hours (I'm currently strapped in as I type lol). On days that I do extend, I usually do it for only 3 hours (two 1.5-hour sessions).
My results...my length has gone from 17.3 cm to 19 cm (BPEL, which means bone-pressed erect length). I'm thrilled!! The interesting thing is that most of these gains happened within the first 400 hours. Not much gains since then, but I will continue to do it b/c I think more gains can come. My girth has not changed at all FYI. Hope this helps!