It depends.

The fastest way is with the people movers from D --> A and then walk down to B but the timing for that is entirely reliant on how long until the next one departs. Once you get on the people mover it should take no more than 5 minutes to get from D --> A and 5-10 minutes to walk to the B gates from there.

I would plan for it taking 30 minutes but it could take far less time if you have great timing.