It takes approximately four hours for JELL-O to set. There are also ways to speed-set JELL-O to make it set faster. Speed-setting makes it soft-set in 30 minutes and relatively firm in approximately one and a half hours.
To speed-set JELL-O, stir ¾ cup boiling water into the gelatin until the powder is dissolved. Mix in ½ cup cold water and enough ice cubes to equal 1 ¼ cups. Mix the cold water into the gelatin mixture and stir until it begins to thicken. Remove any of the ice that has not melted, and place the JELL-O in the refrigerator until it becomes firm.
Add ¾ to 1 ½ cups fruit when the JELL-O is soft-set and place it back in the refrigerator for approximately four hours to finish setting. Do not use fruit that is acidic, because it prevents the JELL-O from setting properly. Flavored soda or fruit juice can be used in place of cold water for the recipe, but soda that contains too much acid may cause improper setting.
JELL-O becomes firm because it contains gelatin, which is a gelling agent that causes the mixture to thicken. Gelatin is used in a number of food products as a thickener to give foods a firmer texture. Some foods that contain gelatin besides JELL-O are gummy bears, jelly, cake frosting and marshmallows.