Just from my experience:
It should start bubbling in anytime between an hour and 8 hours. It is easy to get yeast that is not very active or completely dead, or to kill it with extreme temperatures, or by letting it get moist (high ambient humidity will do), and then letting it dry again.
Placing it under the tank risks water siphoning out of the tank, into the system, and out to your room. You can prevent this if you make sure there are no leaks, however small.
Placing it under the tank risks the yeast siphoning into the tank. In my case that meant completely tearing down the tank, because the yeast, no matter how many water changes, started a cascade of events that left my tank completely covered in a grey slime.
What I do now: Triple check that there are no leaks. I put the whole thing underwater for a full day to make sure no bubbles come out except at the output tube, and place the system under the tank.