The time for an area reset, or repop, varies between each individual area as well as the presence of people in an area. The message for an area reset also varies from area to area. As you become familiar with each new area, you will learn what message indicates repop. Objects on the ground will not be regenerated if anyone is in the area when it resets.
Repopulation (or Repop for short) is the process through which an entire area is regenerated. Mobs are recreated, the economy is reset and area pops are placed in their original positions. Repops occur in specified (but somewhat random) intervals. For example, the repop in Seth's Fortress takes about forty-five minutes, give or take a few minutes. The most common repop time is 15 minutes, as this is the default when a new area is made.