second that. Especially girth gains bring a tremendous amount of volume or to be more precise: twice as much as length gains, since you grow in two dimensions (width and depth). That's also why 1,5" in girth are much harder to gain than 1,5" in length - twice as hard and lengthy one could say if you aren't individually predisposed to gain girth very fast or to gain very slow in length. 1,5" in length in one year are doable if (like Zam said) you're dedicated and set up a good routine. 1,5" in girth I don't consider as doable in one year (more like two years and that would still be very good!). Everything has to fit if you want to gain that fast and that doesn't happen very often (evironmental influences and organsimic reactions you don't and always can't reckon with are crucial factors here as well) . So to be more realistic and to avoid discouraging disappointments I'd set myself a time frame of 3 years if I were you. But as Asanon stated you'll be amazed about the difference 0,5" each will make!
Also (and because of the aforementioned aspects= the growth process is everything else than a continuous development, thus making propositions like "You can gain x inches in x months" almost always an outright lie.
Moreover I'd recommend you to reduce your body fat. For an overweight guy it's a very feasible way to get some length without having to rely on stretching alone. Besides it would be very beneficial for your overall and penile health in general.
btw: the term "lig potential" refers to the maximum elongation potential of the suspensory ligament that attaches your penis (which is an extremity) to your body. It's one of three ways to gain length by stretching and it usually is the easiest and fastest way.