How fast does pork fat spoil?
This is an unwise practice.  While pure fat is actually fairly stable, you don't know how much residual water and other components remain in your rendered fat.  You should collect it and refrigerate it (it should last at least a week, probably much longer in practice) in an sealed and air tight container (to minimize rancidity) if you intend to reuse it.

Otherwise, you should discard it immediately.

For the cannonical answer and reasoning, see:  How do I know if food left at room temperature is still safe to eat?