Another option would be to close voting on a node after a certain period of time (say one month) and show the reputation to everyone after that.

I actually like that threads in perlmonks are not locked after a certain period of time. I have many times found nodes over a year old (the oldest was 2 years) that have been very helpful to me and have upvoted them because of that, one of my own nodes (OT: The WarriorMonks) still gets the occasional reply and/or ++/--, the replies I care about much more than the votes but it's nice to know someone looked at it and liked it. Also, I don't know if you meant all nodes or just certain nodes, but I'm definitely for allowing people to vote on certain long term nodes, like Tutorials or Reviews, for as long as they wish.