Note: Now that Workflows are live in all accounts, you can do everything that Triggers and Campaigns do (and more!), all in one builder! Click to learn more about Workflows.
The Stale Opportunity Trigger allows you to define a timeframe after which an opportunity is deemed "stale", and you can then trigger a variety of actions for that opportunity.
Note - The Stale Opportunities trigger does not fire retroactively but does fire on the multiple of the duration days: