In terms of boiling times, the rule of thumb we were taught is that you pour it boiling over black tea and steep three to five minutes, and for green tea you wait ten to twelve seconds after you've taken the kettle off the heat before pouring it over the leaves and steep four to six minutes. White teas steep for like a minute or two, herbals for upwards of five minutes because they're not really tea made from tea leaves, they're things like tree bark and spices and they take longer to soften and release their flavor.

But really, that's what the directions are for :P And I'd always follow those over any rules-of-thumb I may have tucked away.