Directions for use will vary greatly depending on the acne situation. For example, I have persistent nodulocystic acne that never goes away so I have to take minocycline every day. I have been taking it for almost 8 years now, so duration can also vary.
A note about minocycline: it can cause permanent yellowing of the teeth if you take it while you are younger and your teeth are still growing. This yellowing cannot be changed through any known teeth whitening methods, so use the medicine as little as possible if you are younger and you are worried about that side effect.