if kept in water with no food source bacteria will start to die off after 12 hrs, however if there's fish in there there's always gonna be a relativley steady level of ammonia to keep it going.
if your stats dropped to 0 after fishless cycling and you werent able to add any fish within the next 12 hrs then there's a fair chance you'd have to cycle the whole thing again, if you added fish then they may have to go through an ammonia and nitrite spike.
if your near the end of your cycle and concerned about it, then when you check your levels in the morning and they're 0, and your not sure when you can get fish, just add a little pinch of fish food each day until you get your fish which should keep it ticking over.