Someone somewhere will feel obliged to tell you that it's not advised you should freeze tetras (though I really find it tough to believe it does any damage to the nutrients in them).
But I've always made ice cream from my tetras. Your freezer may vary but I find three hours in the freezer is about right.
Alternatively, freeze as long as you like, but get it out of the freezer about an hour before you want to eat it.
Chocolate ones are the best. Banana and strawberry ones don't seem to work as well.