You should find the time of the free fall (assuming, of course, that the lemming is spherical and in a vacuum). To do this, consider the horizontal and vertical components of the velocity independently and note that the distance traveled from rest in time with acceleration is . Then multiply this time by the horizontal velocity.

This can be found either by finding the horizontal and vertical components of the landing velocity and using the Pythagoras theorem, or from the law of the conservation of energy. You know the initial energy, which is the sum of the kinetic energy of the horizontal motion and the potential energy due to being on the cliff. At the moment of landing, the lemming has only the kinetic energy, from where the speed can be found.