Update - found a nice FAA publication on the conscious time after decompression. They are somewhat more generous than my estimate. Page 13 of the text for those interested.

http://www.faa.gov/pilots/training/a...%2061-107a.pdf

No permanent damage though would result unless very long exposure >30 minutes to these altitudes. So, if plane started down, would rapidly regain consciousness when cabin altitude dropped to 18,000 feet or so.