The length of the track is not actually a prescription. 40 minutes of technology-boosted meditation is rather a lot. Now you may not hear that from the regulars here, because we enjoy it and meditation is a rather big part of the life for some here. But actually 20 minute is more the normal time to meditate for people who are not monks or nuns. Studies that show significant stress reduction and improved health typically use 20 minute meditation sessions.
The CDs are actually 60 minutes per track! The 40 minutes is more of a compromise with memory limits in old MP3 players and dial-up connections, I believe. Having them even this long is typical of Michael's generosity, making sure that even the serious seekers have something to chew on. You are not obliged to clear the plate.
But I agree that if you have time on your hands, it is certainly a worthwhile experience to simply observe your restlessness, discomfort or boredom with detachment, as if you were a scientist from the spirit world observing the emotions of the human subject through a direct visit in their head.
Generally though, 20 minutes is fine. 40 minutes is also fine, of course. I am not sure where the limit is, but I'd take the length of the original tracks as a hint. I play LF2 for about an hour in the morning and have no side effect apart from feeling a bit dazed afterwards, but then I usually feel a bit dazed in the morning anyway.
