Recent developments in processor, memory and radio technology have enabled wireless sensor networks which are deployed to collect useful information from an area of interest. The sensed data must be gathered and transmitted to a base station where it is further processed for end-user queries. Since the network consists of low-cost nodes with limited battery power, power efficient methods must be employed for data gathering and aggregation in order to achieve long network lifetimes. In an environment where in a round of communication each of the sensor nodes has data to send to a base station, it is important to minimize the total energy consumed by the system in a round so that the system lifetime is maximized. With the use of data fusion and aggregation techniques, while minimizing the total energy per round, if power consumption per node can be balanced as well, a near optimal data gathering and routing scheme can be achieved in terms of network lifetime. Several application specific sensor network data gathering protocols have been proposed in research literatures. However, most of the proposed algorithms have been some attention to the related network lifetime and saving energy are two critical issues for wireless sensor networks. In this paper we have explored general network lifetime in wireless sensor networks and made an extensive study to categorize available data gathering techniques and analyze possible network lifetime on them.