With time-based persistence frequency, the server stores session information to the persistence store at a constant interval, called the reap interval. You specify the reap interval under Configurations > config-name > Web Container (Manager Properties), where config-name is the name of the configuration. By default, the reap interval is 60 seconds. Every time the reap interval elapses, a special thread “wakes up,” iterates over all the sessions in memory, and saves the session data.
In general, time-based persistence frequency will yield better performance than web-method, since the server’s responses to clients are not held back by saving session information to the HADB. Use this mode of persistence frequency when performance is more important than availability.