Select all the layers.
Either right-click and select Keyframe Assistant>Sequence Layers, or go to the Animation menu>Keyframe Assistant>Sequence Layers.
Select overlap and put in the interval you want them to overlap by.
Or you can write a script that loops through the number of layers and sets the inPoint of layer[i] to i – something along the lines of
for (i=0; i<=composition.numLayers; i++) {
composition.layer[i].inPoint = i;
}
Note, the syntax on this is most likely all wrong, but this is the general idea.