I suffered a concussion in a crash in July 2010. I was off the bike completely for six months (in fact all exercise except walking) and only back on the trainer for the first four months, working up from 20 minutes easy at the start. I was lucky that we have good concussion specialists here (thanks to hockey and speed skating). I may have taken the doctor's advice too seriously but my concussion was severe. I have no memory for six days after the crash and my son who was with me in Italy at the time says I'd talk for five minutes about something then go right back to the start. I tried working part time after about a month and wouldn't start back that quickly again. I could tell when the weather was going to change for the first two winters after my crash and was relieved last year when I finally couldn't tell when a Chinook was coming.
During my recovery, I got a lot of information on concussion from my physiotherapist (seeing him for the pins in my back but chatted while there) since he works with national speed skaters who, particularly the short trackers, suffer these a lot.
Here's a link to a simple test they use when someone crashes and regularly thereafter.http://www.cces.ca/files/pdfs/SCAT2%5B1%5D.pdf
By the first time I did the test was about three months after my crash and I think my score was in the 25-30 range. The physio told me that 100 at speed skating event for someone who's just crashed is not unusual. Even though I was only at 25 or so, the concussion doctor advised caution.
As someone said, medical advice is important and if you can find a doctor with concussion expertise, that would be great.