It's always once the script's done in the first two years if it doesn't get going somehow or another, I've never had an old script that someone's made later on.
In my own experience, the scripts that I wrote, if they didn't go within two years and become a film, they never went and no one ever came looking for them.
Because I've been at it so long and very steadily, I have a lot of credits, but I probably have twice as many scripts that were never made for whatever reason.