brain tuner

I've been wondering how long at a time people use the brain tuner for?
I know Bob Beck advised not to use it for any longer than 40min at a time but I've been wondering if it's fine to use it for longer.
I also run the brain tuner frequency through rife and bean for long periods but it might not be as effective as directly using bob beck device.
I feel like in terms of been able to dowse effectively, the brain tuner frequency has been the most helpful to me over the years.