I would say that two months, certainly makes someone inactive. One moth: stuff happens, people move, internet gets shut down, and power grids go out... it happens. That's life. 2 months, however, is a pretty sure sign of abandonment. I mean, if the boards are important enough to you that you became a judge, then I think you can find time in a 2 month span to visit a library or other place of internet capability to post SOMETHING saying that you're still alive and well.
As for the votes themselves: I think if nothing has been posted to a proposal for an entire month, then the proposal should auto-fail. If it's important enough for you to propose something, it should be important enough for you to at least bump the proposal once a month while the judges make up their minds.
So here is my proposal:
Votes that a judge makes in a still-open proposal are rescinded if that judge has not posted anywhere in two months. In addition, any proposal that has not had a post in greater than 1 months time (from day X of month Y, to day X of month Z) should auto-fail. These proposals can be re-proposed one month from the date of auto-failure (on day X of the month following the date of auto-failure).
/proposal
Even if something auto-fails, if it's important enough to the proposer that something NOT be included in our house rules (or changes be made to those rules, since pretty much everything is automatically in after only a month due to the new proposal system), they can propose it again in only a month.