Yup, I've heard it as well. It's very incorrect (WAY too high!), but being incorrect has never been enough to kill old wive's tales and urban legends...
The loss depends on the cable conductor size, the dielectric properties of the insulators, and a few other factors that only nerds like me really care about. But ultimately, for low impedance balanced mics and well-shielded cables, you have a lot of leeway lengthwise. You always want to keep things as short as possible, but I routinely work with 150' runs in live recording, and up to 300' in some fixed-installation rooms where the snakes have to take a very circuitous path in their raceways from the stage to the booth...
You really want to keep balanced low-Z mic runs below say 500', but that's hardly a problem for us basement recordists.
Now, single-ended high-Z stuff (like guitar pickups or piezo transducers) are a wholly different story- but you knew that, anyway. Those you have to keep as short as possible to keep from rolling off the high frequencies because of the cable capacitance, and to keep noise induction in the shield under control (since the shield is one of your two signal conductors for single-ended use).
