The primary classes of hormones present in all kelps are auxins and cytokinins.
If, depending upon the species used and processing, auxins predominate, in a matter of weeks you should see the emergence of new root tips, branching from existing roots. If the cytokinins are present in greater quantities, then you will likely see a growth spurt, but that won't be as obvious as soon. If the two are about even, it's anyone's guess.
Then there is the dosage issue. Below some concentration, it won't work at all. Above some other, you will end up with deformities in flowers, and possibly plants.
As far as application frequency is concerned, while the occasional 2-3 waterings in a row can be used to "kick start" a rootless plant or stubborn one, the response to such chemical stimulation can last anywhere from 14-21 days (+/-), which is why I recommend the easy-to-remember monthly application to avoid over-stimulation.