Data aggregation in Wireless Sensor Networks (WSNs) has attracted wide attention since it could reduce the amount of wireless data communication and thus save power consumption, which is one of the top concerns in low-power WSN systems. Given a Data Aggregation Tree (DAT) for data query purpose in a WSN, and the expected query accuracy, we show the feasibility to use a low-cost timing control scheme to adaptively adjust the waiting period in each DAT level so that the end-to-end query delay could be optimized, i.e. without unnecessary pause in an aggregation node. For such an adaptive timing control scheme, we propose to use a Finite State Machine (FSM)- based auto-feedback control algorithm to adjust the waiting period of each DAT node to "timely" respond to the queries. Our sensor network simulation results and hardware experiments verify the validity of our proposed timing control scheme.