[kentich]

> Having the window go black would drive me insane.

It is too easy to ignore the timer, that's why I think forcing breaks is a good idea. In the app I built for this, you can remove black screen if you really want but in an inconvenient way so that it is not too easy.

[al_borland]

Do you at least give yourself a warning, so you can find a natural stopping point? Maybe flashing the screen 2 minutes before the time is up, and then fade to black over those 2 minutes.

How easy a timer can be ignored depends on how annoying the timer is. If it's loud and doesn't stop, it would be hard to miss.