What is the screen life of an Android Tablet?
Sorry, this answer is slightly off-topic but still important for your use case.

The thing you are most likely to have to worry about is heat.  Having the screen on will generate heat, and you will need to plug into an external power source which when charging the battery will also generate a lot of heat.  This could damage the internal components (including the screen) over a long period, but unfortunately I don't have any evidence to support this.