At the concussion clinic I was at, they endorsed the use of free weights and some nautiluses machines. The goal was to use a weight that would allow you to do 10-12 reps that pushed your limits but you could handle.

For me, I enjoyed the lifting immensely but it proved to be too much as my muscles toned and I started lifting more weight.

Trial and error... your body (e.g. symptoms) will tell you when you've done too much. That's the only yard stick that is relevant.