Recently I have reached one of my weightloss goal. At this point regular exercise is not a pain, but a possibility to do it every day. I want to do it regularly so I searched for a software that warns me if I use my computer too much. I discovered Safe Eyes, which primarily focus on the health of our eyes, but you can make custom messages for yourself. It is open source and Linux only though.
Safe Eyes sits on your panel and casually warns you if a break is coming up. The break is full screen, but you can skip it (highly not recommend it, since it goes against our goals). The settings is simple to set up different message.
You can setup shorter and longer breaks after 15 and 75 minutes, default. However I recommend to modify it to 10 and 40 minutes. In this way you will become more active.
It has plugin support for system notification and sound alerts, and you can see your statistics at break screens.