That will greatly depend on many, many factors. If the tank has been set up for a while, and nothing else has changed other than the adding the fert cycle, then it should be relatively soon (although seeing a difference in growth is sometimes difficult to do over a short period).
If you recently made some other changes, that will affect the speed as well. It also depends on what types of plants you have and the conditions that they are kept it (filtering, lighting, substrate, how long they have been planted, what else is in the tank, CO2, etc). Unless you really want to list the exact conditions of the tank and all the parameters from testing, there really is no way to tell. Even then, its a ball park guess.
I recently re-scaped about 6 weeks ago, and other than that, kept everything the same. I haven't seen noticeable growth in anything but my tiger lotus and anacharis up until a week ago. I have ~8 other different plants in there too.
The thing with plants is that sometimes they grow at extraordinary levels, and sometimes it's hard to see any growth at all unless you compare over a larger period of time (weeks, months). HTH, and GL
