My own homespun theory (based mainly on my experience) is that there are basically two phases to recovery:

1. Physical and mental rest (no exercise) until symptoms subside sufficiently (not necessarily completely)

2. Gradual increase in physical and mental activity as tolerated (i.e., so long as it doesn't trigger or exacerbate symptoms)

How long each phase lasts will vary for each person / injury. I think people sometimes conflate the two, thinking that they should be exercising when they should be resting (first phase), and possibly being too inactive during the second phase.

In any event, only the concussed person really knows how much he or she can handle. It's an ongoing trial and error process that requires both adding and subtracting activity as necessary.