The key would be to make it short enough that you can be 100% confident the next person will take over and not leave you holding the bag forever. (Hi Rico!)
I've no idea where the magic number would fall, and it's probably different for every team. I'm reasonably confident it's somewhere between 1 week and 6 months though.
I think one potential benefit from this comes in the form of killing one of the other thngs that Zane mentioned (which I totally agree with): The "(s)he hath spoken" part. I can imagine getting to the point where not everyone on the team remembers exactly "who is PTL this month" and certainly the long tail of minor contributors would lose visibility into this. I think that would dis-empower (in a good way) the PTL-this-month person both from the perspective of being the constant (even if unnecessary) decider, and the sounding board for everyone trying to push their pet efforts through the works. Although I'm calling "not it" for the month containing the PTG right here and now ;P --Dan