Quorum consensus protocols execute requests in parallel and proceed once K out of N responses are received. The performance of a system depends on the value of K, the distributions of the quorum response delays, and on the use of system resources by the N concurrent requests. An analytic approximation is given for the delay to achieve the quorum, which also accounts for the contention and delay caused by the (N - K) delayed responses. Compared to simulation results, our approximation gives reasonable accuracy; about 5% in most cases. Our method is shown to be rapid and scalable.