3 minutes, actually.

Double Dutch said that you have 10 min on easy and 7 on medium, and you don't die when time runs out. On hard and turbo, you do. You get 5 min on hard, but I think there are hourglasses.

Medium on JJ2 is normal.
Easy on JJ2 adds more carrots.
Hard adds more enemies, and changes some of their attacks:
Helmuts jump at you, like the lab rats.
Uterus at the end of "Marinated Rabbit" has 8 orbs surrounding him, 6 on Medium and 4 on easy.
It gives you the most points. ^_^ I scored around 870,000 on hard Single Player.

I prefer Medium.