Any amount is potentially dangerous if there are fish present, though most 'cycling fish' should be able to withstand the nitrites if they are controlled. (I'm assuming you are cycling with fish since you're inquiring about danger levels). If you use Prime to condition your water, you can also use it to 'detox' the Nitrite in an emergency as per the bottle's directions.
If you are worried about your fish suffering from brown blood disease, you can also use aquarium salt to try to hold it off.