The normal schedule is every 20 minutes. More often during heavy loads.
Given the shorter distance, fewer buses would be needed compared with Animal Kingdom going to and from other resorts.
Actually buses do not always go back and forth. During peak hours, only one direction is full, namely from resort to park in the morning. A bus that arrived from, say, Contemporary and dropped off at Animal Kingdom might go to AK Lodge (empty short leg) and pick up AKL guests for Magic Kingdom (full long leg) then Contemporary (empty short leg) then AK (full long leg). The short full legs (AKL to AK in the morning) probably use back and forth buses.