Just dose up to 5 ppm and turn your back on it for a few days. Unfortunately, the bacteria that tend to dominate the colony at 8 ppm are not the ones that we are trying to grow so a high ammonia dose is really counter-productive. If you have a day or two for the ammonia to process down to zero, the bacteria die back from not feeding after that will be quite small for just a few days. Another option, that is more difficult to predict is to feed fish food to the tank as if there were fish in it. The rotting fish food will supply some unknown amount of amonia to maintain your cycle for several days. The trouble is you will have a cleanup to deal with when you return home.