Actually, most "sales" fares are loaded monday night, so the best time to buy a ticket is shortly after midnight on Monday - so Tue AM eastern time.
If the "sale" gets matched, it'll be available until the end - from 3 days to 1 week, generally. If the fare doesn't get matched then many will rescind the sale rather quickly - by noon on Tuesday.
If you look at kayak.com, they'll give you a graph of the prices so you can try to judge what's going to happen.
