Hi oldbones and welcome to the Project Meditation community.
No, you don't have to meditate for 40 minutes, but 40 minutes is supplied for those who wish to meditate for longer (I think it's 60 minutes on the purchased CD's).
You can either set yourself an alarm or just meditate for as long as you feel comfortable. Some people also set the track to play from 20 minutes in, so that it finishes by itself 20 minutes later, which is also fine as the entrainment frequencies are consistent throughout the whole track. There's no need to begin at the start of the track or finish at the end of the track if that doesn't suit you.
Hugs
Giles
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