The bus-ride from PI/West End to Contemp. is about 10-15 minutes on the road.
But you must wait for a bus to arrive.
I'd say to allow 45 minutes to make the trip.
Add to that the time it takes to get from the bus up to your restaurant in the resort, plus you want to get to the front desk at the restaurant 10-15 minutes before the ADR time... so you end up with the "standard" Disney Transportation answer, "Allow about an hour to make the trip."
That will account for bus waiting, passenger loading, if there is wheelchair-loading on your bus, slow traffic, etc.
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