The cycle will go as long as it needs. So you need to add some type of ammonia (this can be a piece of food, or ammonia solution), now people frown on using fish to cycle the tank. Anyways, the cycle will go on until your bacteria in the tank is built up sufficiently to go through the nitrogen cycle of ammonia > nitrite > nitrate. So people starting their tanks will test for these to see when each will go down. When the cycle is done, you will detect zero ammonia and nitrite...or at least very very low. I would add in a big of live sand or rubble from another reefer to kick start the bacteria. Other also add in bottled bacteria to speed the process.
Or you can just wait it out, and keep feeding to get ammonia into the tank, and wait a month. That's usually how long it takes.