For a motor with constant rotation, high-speed rotation and/or rotation in excess of about 350°, your standard pot will never hold up. I'd give it no more than 24 hours of operation before something craps out. I agree with with Alec that a shaft encoder would be the way to go. It's already digital, so there's no need for an ADC; it's in Grey code, so there's no ambiguity in positioning; it can handle any speed, depending on which encoder you buy. You can roll your own encoder with a good drafting program like AutoCAD, providing as many bits of resolution as required. And making your own can eliminate any shafts within a bushing that will limit rotational speed and MTBF, as there will be little-to-no frictional losses. The most difficult part would be machining your own optical pickup bar, but that can be a fun project in itself!