Definition
Power reserve is how long, measured in hours, a watch will run once its been fully wound. Typical values are around 40 hours for a single-mainspring movement like the ETA 2824 and 50 hours for a high-end automatic like the Seiko 8L35.
At the extreme, there's the Lange 31-day movement, costing tens of thousands. 8-day movements are common on high-end Panerais and others, and twin-barrel movements are often good for 72 hours.