In cognitive radio (CR) networks, licensed spectrum that can be shared by secondary users (SUs) is always restricted by the needs of primary users (PUs). Although channel aggregation (CA) can enable each SU to utilize multiple channels at a time, whether it is beneficial is subject to PU activity and radio capability. In this paper, we study the efficiency of CA in consideration of various such practical constraints and costs. First, we propose a new channel usage model to analyze the impact of both PU and SU behaviors on the availability of white spaces (WS's). This model is very general and can capture a wide range of user behaviors. Next, we model the delay costs for performing CA. User demands in both frequency and time domains are considered to evaluate the costs for making negotiation and renewing transmission. Further, an optimal CA strategy is defined in order to minimize the cumulative delay for transmitting certain data. Numerical and simulation results based on real data of PU activity show that user demands on both aggregated bandwidth and service duration should be carefully chosen in practice.