Crosby missed from Jan 5, 2011 to Nov 21, 2011 with a concussion, then was concussed again Dec 5, 2011 and returned March 15, 2012. Then nothing until he was concussed in Oct 2016, returning to action on Oct 26th. He then was concussed in the 2017 playoffs during game 3, returning for game 5 of the same series.
My point is that Crosby missed anywhere from a few days to nearly 11 months with concussions. There's absolutely no telling how long recovery will take - that's a tremendous amount of varience with the same person during his recoveries.
All I can say is I feel for Foley, I can't imagine how difficult dealing with prolonged concussion issues would be at his age.