In addition to what everybody else said, a lot depends on the size of your tank and how many fish you've added.
Measurable levels of ammonia will occur more quickly if you have a small tank (smaller volume dilutes ammonia less than a larger volume). Similarly, they'll occur more quickly if you've added more fish (more fish produce more ammonia).
If you think of it in terms of a concentration problem (density = mass/volume, so concentration of ammonia is proportionate to total number of ammonia producers [otherwise called fish] / volume of tank) it's easier to visualize.
Since everybody has a different set up, it is safest to test the next day after adding fish, and each day after that.
However, the best and safest way to cycle a tank is by cycling fishlessly.