Power reserve refers to the amount of time that a watch can run without winding.
Most watches can run 40 to 48 hours without winding. This is seen as sufficient for all-day wear and to still remain running in the morning.
Running time is restricted by mainspring technology. Longer springs tend to cause a watch to run faster at first, then much slower. This has limited running time without the use of a special regulation system, as with the Lange 31 or the DeWitt Academia Tourbillon Force Constante, by which the driving force of the mainspring is held continuously equal during the entire running time.