I use Google Reader and on my to-do list is to hack together Perl script using WebService::Google::Reader [1] which is based on the "unofficial Google Reader API" [2]
It would look at my subscriptions tagged with "discussion" and where the difference between now and the "published" date was over a certain period it would automatically unsubscribe.
You could probably do some neat stuff with using different tags to specify the length of time you wanted to subscribe for, etc.
I’ll update here if I ever get around to it, but I wouldn’t hold your breath. If anybody else takes the idea and runs with it I’d love to see some code.
[1] http://search.cpan.org/dist/WebService-Google-Reader/lib/Web...
[2] http://code.google.com/p/pyrfeed/wiki/GoogleReaderAPI