To estimate the packet delay in the Weighted Deficit Round Robin algorithm, we consider an M/G/1 queueing system with multiple vacations and the E-limited service discipline. We give two different approaches: In the first approach we obtain a closed form expression of the mean packet delay, and in the second approach we obtain an approximated formula for the mean packet delay which is simpler to compute than in the first approach. Numerical studies indicate that the exact value and the approximation value are acceptably identical. Some further discussions on the mean packet delay are also provided.