Live rock and/or sand help speed things up because they have the beneficial bacteria already in them versus starting from scratch when you have just regular rock and/or plain sand.
The thing that keeps a cycle going is ammonia. If nothing is in the tank to add that, them more BB are not being produced and can actually begin to decrease.
The best way to know for sure if your tank is ready for fish is to dose with ammonia (I've heard True Value Hardware sells it). Get it up to 4 ppm
and see if it gets down to zero within 24 hours. If so, you can add fish knowing that the ammonia and nitrite levels are going to disappear rapidly.