Alot of people say that unless you spend like two weeks on each bargue plate then its pointless, but then wouldnt it have taken like 3 years for van gogh to do each one twice? and didnt his brother help publish it? i feel like he knew how to do the exercises properly.
I think the idea is more the requirement for care to measurement and precision rather than any arbitrary time-limit. Basically it means "don't rush it and just slap down lines until you get lucky, take your time and make sure you get it right intentionally."
That's my impression anyway, I'm not super well informed on bargue plates so take it with a grain of salt.