Area resetting time varies from area to area. The length of time is
determined in the area file by the builder. Areas like the training temple
reset every 3 minutes while major areas like Waterdeep take around half an
hour.
Also note that objects lying on the ground will not be regenerated if anyone
is in the area when it resets.
Area resets are indicated by a special area or sector specific message just
prior to the reset, something chosen by the builder. It might be "You hear
the soft murmur of voices," or "Everything grows still for a moment," or
something else vaguely along those lines.