Yes, it's possible to bf on a schedule, as these posts show. But in the early days the research shows that following the baby's cues is more likely to stimulate the milk production adequately, and keep the baby happy. This is pure physiology.
Feeding for hours and hours is not normal . A lot of babies have the occasional feedathon, but I mean every day, feeding, feeding, feeding with the baby never coming off full and drunk. This is ineffective feeding not demand feeding! It's often amended by attention to positioning and attachment, to allow the baby to remove the milk better.
GF is wrong to imply that all babies will get to hindmilk after X minutes. This is very variable. Best to watch the baby, not the clock, anyway. In any case, there is no massive cut off point at which the milk changes in the feed....it's more gradual. There may be several let downs per feed, and many women do not feel them ever! Many women are not full between feeds, especially if the feeds are frequent. This has no bearing on overall production at all.
The effect of diet and rest and weight loss is not as great as you'd think, though of course individual women may react in individual ways.
If it's important for how someone wants to do things, then a sort of flexible scheduled feeding can work ok with a baby who is gaining weight and is happy enough...but it really is not a good idea to start off this way. Just ask any breastfeeding counsellor who takes calls from mothers distressed and confused (and short of milk, sometimes) who have been desperately trying to follow the book.