Is there a guideline for #fish per gal to start cycle?

[gfink]

I found lots of information about cycling tanks, but no guidelines as to how many fish you should use to start your tank. I can see starting out with 1 or 2 in a small tank, but what if you were starting a 55 gallon?

Background:
I have a new 29 gal tank. I bought 5 Zebra danios to start the cycle. I chose them because I read they are resilient. I chose 5 so there would be enough to form a small school.

Question: Should I stick with this number, or will the cycle take forever since they are so small compared to the tank volume?

FYI after 1 week, ammonia still reads 0 ppm.

 

[BrianNY]

Be patient and keep watching for an ammonia spike. It will happen. You've got enough fish to begin the cycle.

 

[FawnN]

I've only cycled with fish once until I found out how much easier it is to do a fishless cycle.

But from my experience cycling with fish, it takes about 1.5 to 2 weeks before you get an ammonia spike.