You can sleep on your stomach until it is no longer comfortable for you.
With my first I slept on my stomach until I couldn't anymore, meaning uncomfortable.
I asked my dr at 9 weeks and he said to stop now, it does start to get uncomfortable and eventually u will forget about laying on your stomach. After my first I went right back to sleeping on my stomach now at 12 weeks I still want to lay on my stomach but I use a body pillow to try and make my night comfortable.