This is all down to what sort of tourist you are.
If it's beers in tourist bars and wandering around a few temples mentioned in Lonely planet, 5 days is plenty.
If you want to see the country, two weeks to a month is going to give you a fair idea of the place.
That applies to pretty much every country in Asia, save perhaps Singapore, and only because you could walk all the way around it in a day or two,