It's hard to give a rule of thumb for time, But a small load like 10 zebra's in a 55 will cycle a tank a little slower. Although it is always frustrating to wait, I reccomend any time you do a fishy cycle you take your time and do it slowly, also when you add more stock do it slowly as well. Once the bacteria is estabilished, it still has to multiply to account for larger bio-loads. Daily testing is important, and if Ammonia or Nitrite levels start to rise, they should be controlled with water changes to keep your fish happy. There are some hardy fish (danios included) which tend to be less sensative to fluctuations, but it is not fun for any fish to go through the cycle and ammonia and nitrite will kill any fish at some level. I am not familiar with the bio-Spira product except that I have seen it recommended enough on this site to think it probably helps. Anything that makes the cycle easier for your fish is a good thing. If you are in a hurry I would find out more about Bio-spira and try it, A "live" filter sponge, or substrate from another working tank will provide similar results by adding bacteria rather than waiting for it to grow.