It depends actually. If I'm angry over something someone did in respect to me, sometimes for minutes, sometimes for hours, usually never more than a day though, and then I move on and let bygones be bygones.
When I think about it I tend to forgive others much more easily than I tend to forgive myself.
But, if the somebody hurt someone I care about, and that's why I'm angry, my anger can very well last a lifetime, cause I find it hard to forgive what is done to those I care about. I can only do it really, if I'm convinced the person I care about forgives the other one himself.
I can cool down my anger to zilch by picking a situation apart at times, if I get the chance to take a step back and think things through. Often it only needs a few moments in quiet to defuse the anger and the uproar dies down. It's just my temper flares up so quickly I don't always manage to stay ahead of the heatwave and remove myself from the scene to get those few moments of quiet and cool down.
It's something that takes practice, I guess. But worth trying time and again, cause at least I usually feel rotten whenever I give in to expressing my anger full force. It leaves me drained and upset and unhappy, cause afterwards when I've calmed down I usually come up with solutions that seemed much more appropriate and mature for handling the situation.
I'm not really fond of the emotion "anger" on the whole, it's better than being depressed and defenseless I guess, but caught up in it full force it's difficult to reign yourself in, like trying to wrestle down a furious tiger.