About 20-30 minutes in and out off peak, probably around an hour in rush hour!
Given the 9 or Blue/Green take only 10-15 mins from Sowton and the Red/4 take 10-15 minutes from Honiton Road its is sensible for Falcon not to serve Exeter but the connections need better publicising. Also Falcon drivers do not make it clear that the bus to the City is included in the price and I've yet to find a driver on a local bus that knows that either. Not sure how many Plymouth drivers know that a Falcon ticket is valid to Tavistock/Saltash/Totnes either. From Exeter to Dartington I usually travel via Marley Head now using Falcon and Gold.