It's definitely a sine curve.
Basically you exercise hard and then enter a catabolic period for a day or so. Then afterwards, the anabolic hormones in your body shoot up and the muscles rebuild (although its all continuous). Technically muscles start rebuilding after you stress them, but it's generally a combination of both as catabolism decreases, anabolic processes increase. In any case, the supercompensation for strength gains I believe last for about 2-3 weeks afterwards before you start to lose them.
You guys should read up a bit on fatigue-fitness theory. Here's an excellent series on periodization (even though its a bodybuilding site) w/f safe: