In cognitive radio (CR) systems, spectrum availability changes as the location of mobile users changes. As a result, the band that a CR user is currently using can become unavailable, so the user should check periodically whether the band is still vacant. This checking can cause extra overhead to the system. Herein, we consider channel selection and determine the optimal time interval between two successive checks for spectrum availability; there have been no previous works of the investigation of this problem. In our scheme, we maximize the duration of spectrum availability checks while ensuring that the probability that the CR user disturbs the primary user (PU) is less than a specified threshold. Analysis and simulation results show that our analysis is accurate and spectrum availability can be checked efficiently with low overhead. In addition, by selecting an appropriate channel, the change in spectrum availability due to the change in location of users becomes insignificant, to the extent that it can be ignored.