While I agree with AnthonyGA, a case of fainting, invariably is a diagnosis of exclusion. More interestingly, loss of consciousness due to fainting is found in 12 to 48% of healthy young adults. Common faint, also called as vasovagal syncope, is reported amongst 3 to 37% of the general population.
A study at USAF School of Aerospace Medicine had a referral of 47 cases of loss of consciousness over a period of 8 months, including 34 cases of fainting, of which 24 were found to have common faint.
As far as commercial aviation is concerned, if any pathological cause is ruled out (except in a rare or unpredictable case, which may be missed even on most exhaustive investigations), it can be safely assumed there is no life threatening risk of death. Therefore, there is no reason to prevent an aviator from flying in the event of a single episode of fainting, if exhaustive medical evaluation have ruled out any disease state.
The time for observation 72 hours to 6 months is discretion of the AME or the regulator. If they can define their reasons for the chosen period of non-fliying status, it is prudent to respect the same for it is not just the individual health but the larger issue of aviation safety at stake.
PS: Some of the figures quoted in the mail is taken from an article:
http://www.ingentaconnect.com/search...pe=text%2fhtml