I can't cite the reference but I heard more than once about a military based study where folks going through basic training were given either bed rest or still required to do their physical training while sick (common cold). The results were that performance was reduced while sick but that bed rest did not shorten recovery (and that exercise did not extend illness). The main finding was that the recruits who got bed rest were subjectively happier than those who were required to exercise...

How long to get back to pre-illness form - don't know but you should be able to train through your illness.