I have used an X4 for about 2.5 years though I added a VacExtender mod Kit about a year ago. As a result, i have gained about 3/4 inch In EL. I extended for 3 to 4 hours a day, 5 days a week during the first year. Subsequently, I reduced my time in the extender to 2 to 3 hours a day. Had I been more dedicated, I may have had more success, but I am satisfied with my results. I am a very patient guy.