I didn’t look yet at the Random Extrusion compound Guillaume posted yesterday, because I wanted to work through the basics on my own (but do go get his compound, it’s sure to be pretty useful). Here’s a simple ICE tree that uses a different random length for each extruded polygon:

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You can’t get different extrusion lengths for different polygons without using the Repeat node. That’s because the length, inset, and transform are basically per-object, even though you can plug seemingly per-poly values into those ports.