I wash my hair twice a week, so if you're used to cowashing I would say you can certainly co-wash twice a week. But it really depends on what your scalp is used to. If you're new to co-washing, I would cowash and see how long you can go without washing again. You may realize that your scalp is too oily and needs to be cowashed 3 times a week. Allow yourself a transition period to see what works best for you.
No hard and fast answer. Everyone's hair/scalp and lifestyle/climate is different and these all play a role in how often you will want to wash/co-wash. I can skip anywhere from 1 to 3 days cleansing my hair. I can co-wash 5x before using a sulfate free shampoo and other times I co-wash once before I feel the need to use sulfate free shampoo. It just depends. Only you will know if you start observing what you've done and making tweaks to that. I love that my light co-wash (V05 conditioner) actually gets my dirty/oily hair cleaner than water which doesn't make sense since it's a conditioner. Don't worry too much. Just listen to what your hair feels like needs and you'll figure it out, no prob.