You need to contact your local permitting authority to get an answer...their answer is the only one that matters. In my area of Western Washington, it's a minimum of 24" and you have to trench deeper than that to lay in bedding material to a proper depth. Also, the fact that it is going under a roadway may dictate greater depth or particular specs for the conduit.

As Kudzu alludes, the NEC has for a long time (as long as I've been working), any wiring passing under a driveway MUST be down 24" regardless of what it's in. If the service disconnect is at the pole/meter, then this is not a service entry line but rather a feeder from that point to the structure.

Note you'll need a grounding electrode system (e.g., rods) at both the service disconnect and the structure.

If the service is actually at the garage, then you'll have to discuss with your power company what their requirements are.