I would pick a time frame... something in the rules for posting questions that is clearly defined. I would think that 3 solid months of no answers, comments or additional information provided means they really don't care about getting the question answered. I don't know of many technical issues pressing enough to post on here that I could wait 3 months to figure out. Why 3 months? I think one month is too short and I think 6 months is too long due to my experience working in IT and how quickly the rules change. What do you think? (thanks for your comment by the way.)

using that logic, why bother keeping them around? if you're going to negate the older questions (for whatever reason - unclear, no longer exists, deprecation, etc.) why bother at all? Personally, I find that a bit defeatist.