Layer --> Time --> Time Stretching or you might get there by changing the framerate in the source footage interpretation. Depends on what you prefer and wheter you want to use additional interpolation.
I hade many layers, but I figuered out a way to do it on my own (proud! ). I put my composition into another composition, then right clicked the comp and clicked 'time' -> 'time stretch' and set the new duration to 20. Seems to work good!