With at least 24 hours notice, sessions can be cancelled or rescheduled free of charge (you'll get an automated reminder 48 hours out from your session, so you should have plenty of time to regroup and reschedule, if needed).
BetterHelp's basic plan includes one live session per week. BetterHelp does not set an absolute limit on session length but indicates that most live sessions last 30 to 45 minutes. BetterHelp defines a live session as being either a live video session, a live audio (or phone) session, or a live chat session.