It benefits the project to have a short list of active pull requests. This helps maintainers to respond quickly to active contributions, and helps developers to see what work is actively in progress.
When a pull request has not seen any pushes or comments from the contributor for 7 days, it should be labeled as "stalled". At this time, a maintainer should post a short comment to the pull request asking for a response from the contributor.
If no action has occurred on a pull request one month after it has been marked "stalled", it can be closed. Include a comment suggesting that a closed pull request does not delete the corresponding code, and the original contributor or anyone else can easily open a new pull request with more work completed.
Of course, use discretion in any of these changes -- if a contributor has given a good reason for their silence, the request can remain open.