I think it largely depends on why one took a break in the first place. If they took it because they were bored and didn't really care about the game anymore, it might never come back. If they got hurt, they might just be itching to get back, and no skill will be lost. It all depends on the mind, and the fire in the belly.
With enough 'fire in the belly', the fine-tuning can be back in a day or two. Or even a couple of racks if not off for that long. Sometimes you will even find yourself playing better if you had been thinking about pool a lot while not able to play. The reason for that is that your mind goes over and over the right way to do it. And it gets ingrained. The bad habits can be gone. Unfortunately, most do come back within a few days of playing again if you aren't careful.