I have a 16 gal
aquarium, and I stared off with 4 zebra danios.
My cycle took almost 4 weeks to even begin, and once it did, boy did I notice!! It took another 6-7 weeks once it started to finish it's cycle. I know a lot of people preach patience, but I know the feeling....it's very hard to sit by and wonder when will it start and when will it end.
I take these numbers as averages, not concrete. Just be prepared to do numerous water changes, even possibly daily when the spikes start to happen. I was changing about 10% water daily for a few weeks straight, but the good news is all my danios made it and I still have them today!!
You may want to do a search for Bio-Spira, it's a refrigerated 'real good' bacteria that can lessen the time of a new cycle tank. I was never able to find it when I started, but people swear by it.
Any other 'bottled' treatment other than Bio-Spira that claims it can cycle, I'd be very wary about. It seems most members here would agree those claims are bogus and all you're doing is cycling your wallet