Simmer for 10 minutes, or until the orzo is almost cooked through, and most of the liquid has been absorbed by the orzo. Regularly stir the pot gently, pulling at everything settling at the bottom of the pot, to avoid it from scorching.
If the pasta isn't tender yet, continue cooking with the lid off after the 10 minutes. If most of the liquid has been absorbed, add more stock, a little at a time to keep the orzotto creamy. Better to have just a bit too much liquid, than have the orzotto finish dry and clumpy.
See blog post for more details.