There is no period of time that is short enough. Even a year, three months or a week is too long. Copyright can't keep up with the speed of our culture anymore. The same goes for patents and progress, essentially. It's time to get rid of this artifact of the 20th century (along with the eternally backwards industries around it), it's holding us back.
The shortest time worth considering is a full publishing cycle - from pitch to advance to raw product to publication to break-even to profit. Given all the queues in each of the pre-publication phases and the slowness of making back the cost of preparing a work, I doubt you could squeeze that under 20 years. Otherwise it's just a cruel tease.
If this way of publication can't keep up with reality anymore, then I'm afraid it's not a viable business model, and should be scrapped, not kept on life-support with increasingly ridiculous monopoly laws which ultimately harm humanity as a whole.
