This one is two related parts but feel free to split it if need be.
The first part is a reminder for a running timer either through push notification or through email. Toggl has this functionality and it's very helpful for ensuring time accuracy and that you don't forget to stop a timer.
The second part is a configuration option for autostopping a timer after a defined amount of time so I know that even if I forget to stop it, the configuration will stop it.