This is a complex matter. The amount of time the teabag spends in the hot water is going to affect the tea in at least two different ways: (a) the longer, the stronger and (b) the shorter, the more 'exciting'.
(a) The more time the teabag sits in the hot water, the more it will release its elements in the water creating that strong taste tea has when it is really dark.
(b) Tea contains caffeine; but it also contains tannin, which is a caffeine-antagonist. The release of caffeine starts immediately, while the release of tannin starts after 2-3 minutes. Therefore, a 'short tea' would have stronger exciting effects.
So, my question is: which one do you think it is the best timing for teabags? I know different cultures have different standards, but I do not know if a rule-of-thumb or golden rule actually exists.