(8 weeks) 28 shots of prop in one area may be alot. Don't guess it will kill you but you may want to look for other areas. There are glands in your body (head, armpit, below waist, and feet). These glands remove impurities. The glands closest to the impurities do the work. Also "leftovers" of the shot take longer than 2 days with prop. I guess you can stay in the glute but it would be better to find some other spots as well.
As far as needles, if you go to the doctor and he gives you a correct shot of testosterone its a 20 gauge 1.5 inches. Thats the producers reccomendations for Depo (I've given them). Go with at a minimum 23. Many don't think it matters but trust me when you get that drip coming back out after pulling, your wasting. For much less puncture pain stick the skin quickly, then push.