electrobob.com has build a device that alerts you if sitting to long:
Sitting for long periods has become more common in today’s jobs causing serious health issues. Unfortunately the plethora of activity trackers fail to address the issue: being rather active while sitting does not provide a good picture of the amount of time spent sitting.
Sit.Up is a simple device that attaches to the chair and alerts the user by vibration if sitting for too long while also tracking the sitting time. It aims to be easy to install and forget on any chair and work with any person. On top of that, it is very low cost and has months to years of battery life. With an optional WiFi connection, the data can be uploaded and you can track your sitting times.
Sit.Up – alerts the user if sitting to long – [Link]