For your first cycle, I would not go any longer than 12-16 wks. A big part about cycling gear is about getting good recovery after, so that you can maintain the muscle you have gained. The longer you are on gear, the harder it is to recover. So at any point in the next few weeks, I would suggest stopping your shots. Then give the test 2 wks to clear, assuming you are taking test e or cyp. Then from there, start your PCT. It would not hurt to extend PCT a little longer if you like. My suggestion is usually 100mg clomind for 7 days. Then 14 days at 50mg. If you wanted to run 50mg for a month or even a little longer, I don't think you would have any issues. 

Best of luck with the back. It always sucks to be injured in the middle of a cycle. Remember this. Your back was telling you something. At some point in a lift, you must have lost your form. Keep your sets tight and always pay attention to how you are training. This is a long term game. Lifting heavy is important, but only lift the amount of weight you can control. Slowly over time, you will continue to get stronger. If you find that you have some developmental issuesl with the low back, maybe that is worth exploring. Things like yoga and foam rolling help me a lot, to keep my back healthy and keep me training in the gym