How deep does schedule 40 PVC conduit need to be buried for 30 amp electrical service from house to garage?
It depends. Mostly it depends on what's above the conduit - if there's at least 4" of concrete above the conduit, and it's at least 6" to either side, that's all you need. Combines well with a concrete walkway to the garage, say.
With 2" of concrete on top of it in the trench, it can be 12" deep (to the top of the conduit - 12" of cover.) If you hit solid rock before that, you can also concrete encase it 2" above, to the sides, and down to the rock.
Just dirt, you need at least 18" of cover. Technically you may also need to change to schedule 80 for any above-ground part that is deemed "subject to damage" (a vague part of the code, leaving it up to the inspector) - practically speaking it often makes sense just to use schedule 80 PVC for the entire run - it costs more, but usually not that much more, and it is more durable.