When making Jello it takes around 10 minutes for the Jello to Partially set.
Once you're done putting the Jello Mix into a container then place the Jello into the refrigerator and allow it chill for 10 minutes which is when the Jello should partially be set.
After it partially sets the Jello then takes between another 30 minutes to 60 minutes for the Jello to fully set up and be ready to serve.
So after placing the Jello in the refrigerator allow at least an hour for the Jello to set up and be ready to eat.
If you remove the Jello too soon from the refrigerator it won't be setup properly.
You have to place the Jello into the refrigerator for it to setup otherwise it will not setup at room temperature.