A passive infra-red sensor (PIR) triggers only when a heat source moves across zones of the sensor. So to stay triggered the heat source would have to be constantly moving. You can set up your automations so that the light only turns off if there has been no movement for a certain time. See here for an example of this sort of automation.
See this diagram showing the zones of a typical sensor:
Some sensors are ‘pet-immune’. They don’t detect small heat sources moving down low. Most also have a sensitivity or pulse count setting so that an object has to move though a zone (or multiple zones) more than once to be triggered.
PIRs are not as sensitive at detecting heat sources moving directly towards them (as you are likely to stay in one zone). Mount the sensor so that people move across the front of the sensor and thus cross multiple zones.
I use them effectively to trigger lights in traffic areas (e.g. hallways) but do not use them in rooms where I can be still for some time (e.g. living room).
Be aware that battery powered wireless PIRs will have a minimum on time (from 30 seconds to 5 minutes, depending on the model). This is done to conserve battery life and can interfere with your intended automations.
One last thing, if used outside make sure you have a condition that only allows switching of the light after sunset. The sun moving to or from behind a cloud will trigger them. This is not as much of a problem indoors but try not to install them facing windows. There are ‘outdoor’ PIRs that also have Doppler radio movement detection sensors and both the PIR and Doppler sensors have to be triggered to cause an output. This is supposed to prevent false triggering by the sun. However in practice warm air currents can trigger the Doppler sensor and you still get false triggering.