As soon as the person is out, there is some amount of damage done. It doesn't mean that they are going to wake up a vegetable though. The amount of time it takes to do more severe damage to a person will depend on a number of factors.
The better shape the person is in, the faster they may recover. I'm sure Lance Armstrong would have a faster recovery than the 400 pound couch potato next door.
The longer you keep the choke is kept on for, the more brain cells will be die, and the greatere the chance for permenant damage.
The frequency of times the person is choked and the span of time the chokes are applied will alos be contributing factors to amount of brain cells are killed and how long it takes to recover.
The actual time it take to put the person under would vary from person to person, and also how well the choke is applied. Ive put chokes on some newbs and they may go out quickly, where as I put an identical choke on more conditioned or experinced grapplers and they last longer. Possibly due to better conditioning or simply cause they know how to relax and not burn through the oxygen left in there blood as quickly as the newb.
There is no deffinite or set time, but those are some of the factors I can come up with off the cuff. Hope it helps. TTT Jon