I still often wake up in the night on my back.......my body lets me know when I've been on it too long! I get very uncomfortable, and the tingling and numbness that others talk about. Sometimes bubs kicks me too! I try not to sleep on my back, but it's difficult, I don't like sleeping on my side (I get the sore shoulder too!). I find pillows very helpful, especially one between my legs coming up under my belly. If I really feel the need to lie flat, I just prop half of my back on a pillow so I'm not totally flat IYKWIM? I've heard that this is acceptable.

As to when to stop back sleeping.......I continued to do it until my body told me not to! It was pretty clear when it was time to stop LOL!