Hey there? What do you mean by cycle. Cycling takes as long as it takes. If you mean how long do you let it sit before adding fish then you should definitely read up on fishless cycling. Cycling will only occur after ammonia enters the tank either through fish or through artificially adding fish waster (ammonia) to the tank. Fishless cycling has been wonderful for many many people. I'm currently fishless cycling for the first time with a 55 gal tank.