We describe an opportunistic channel access scheme for cognitive radio-enabled secondary nodes (SNs). The proposed scheme uses the residual channel idle time distribution to estimate the transmission duration in the remaining idle time, subject to an acceptable Primary User (PU) interference constraint. The SN then transmits the frames within the estimated duration without further sensing the channel, which reduces sensing overhead. The scheme does not require the SN to continuously sense the channel to keep track of the start of the idle period, thereby conserving energy.