I've tried to tell me parents we need it up and my mom says "yeah we will get to it" we bought it and everything. I offered to put it up but they won't let me
One on every floor of the house. We had a carbon monoxide scare one Christmas Eve. Turned out that the fumes were caused by my father, who decided it was a good idea to repaint our basement's walls in the winter and not open a window. Since then, we've made sure our monitors are always in good working condition.
(same as the Smoke/Fire Detectors are also hard-wired in)