Session level classification of wireless packet data communications enables differentiated duration-based accounting. In a network offering prepaid services, for example, an applicable accounting policy is identified each time a user requests a new session. Different session classifications are possible under the identified policy. For example, if the prepaid subscriber also subscribes to a push-to-talk (PTT) service, the policy enables classification of each session as either PTT or non-PTT. A different policy would apply if the prepaid subscriber did not subscribe to the PTT service. A network element serving the mobile station during the session monitors packets, until it can detect a packet enabling appropriate classification under the applicable policy. The prepaid platform bases its accounting on the session classification. For a prepaid subscriber with PTT service, for example, a non-PTT session receives normal prepaid treatment based on session duration, whereas packet transport for a PTT session may be free.