Exposure to smoke and carbon monoxide are very dangerous to human health and capable of incapacitating a person in minutes. Smoke will generally be the first warning of a fire in development. Homes and their contents burn more quickly and produce more deadly gases than thirty years ago.
Give yourself and loved ones the greatest chance of survival and earliest exit from a smoke, fire and carbon monoxide scenario. Test your devices monthly and replace the battery (for battery operated devices) at least every year. Interconnected alarms are more effective in enabling you to hear a detector that has sounded in another part of your house.
In many jurisdictions, the minimum requirements in terms of the numbers of detectors and locations of those detectors are just that, the minimum. Do you research and ensure you exceed those minimums, have extra detectors in key locations, and provide yourself the greatest chances of surviving what could quickly become a lethal scenario.