How often should smoke and CO detectors be replaced?

Smoke detectors: replace the whole unit every 10 years (the sensor degrades regardless of battery). CO detectors: 5–7 years depending on brand — check the sticker on the back for the exact replace-by date. If either detector is over its date, replace it now, then set a calendar reminder for the next one.

Where exactly should I put carbon monoxide detectors?

One on every level of the house, one within 15 feet of every bedroom door, and one near (but not directly next to) any combustion appliance — furnace, gas water heater, gas range, attached garage. Mount at outlet height or higher; CO mixes with air, unlike smoke, so ceiling mounting isn't required.