I honestly had this exact same question when we got Nimbus and Storm in May. I was terrified I wouldn't know when they were ready to pluck and we would lose all the fluff and all this fiber farm thing would be for naught and I was going to be an utter bunny failure.
However, I was lucky enough to have a Newfoundland dog already. At first glance, newfies and bunnies might not have much in common, but Newfoundlands also blow their coat. For anyone who has had a regular (non-newfie) dog, you may think you know shedding. Dust bunnies collecting in the corners of the house. Dust bunnies are nothing. We have dust buffaloes. Seriously, some of these things are the size of angora rabbits themselves. Twice a year, Thora blows her coat and will literally lose hair with every step. God help us if she shakes, or sneezes, or there's a stiff breeze. It's everywhere. It's on everything. There's no escaping from it. That's what a blowing coat is like.
For the rabbits, it's a bit more contained because they are in a hutch and outside. I can usually tell when they are ready to be plucked because there is hair gathering everywhere. On the corners of the hutch. On the doorways. On the edge of the food bin. The fluff clumps together on any surface they may have brushed up against. The girls were ready for their first plucking around July, when they were about four months old.
Also, plucking is very different from regular grooming. When I'm grooming, I pretty much just brush out the tangles, make sure there aren't any matts, and trim if they need it (usually in the super snarly between the ears areas.) When I first got the rabbits, I would save every tiny scrap of hair from every brushing. It was a little ridiculous. I stopped that after I did my first plucking and realized that spending time getting out the tiny snarls and matts in every day fluff is inane. These are fiber animals. There will be more wool. Save only the good stuff and move on with life.
This is Storm halfway through a plucking:
The darker hair on top is where she's already been plucked. The fluffy "dust ruffle" at the bottom still needs to be done.
Mid-pluck. You can see how much longer the bottom hair is than the top. I try to pinch a clump of fluff at the base, where her skin is, with one hand and pull with the other. That way only the loose fluff pulls out and I'm not tugging on her skin.
The fluff that came out with that pull. Sometimes it takes a few times in one clump to get out all the long loose fiber and leave her short coat behind. However, if the bunny is ready to be plucked, the loose coat should really just slide out with some gentle tugging.
This is the difference between grooming and plucking. On the left side is what came out during a normal grooming, or brushing, session with Nova today. On the right side is what came off of Storm during today's plucking session. There's a huge difference in the quantity of fiber you get when the rabbit is ready to be plucked. Angoras blow their coats around every twelve weeks, or four times a year, so the girls will probably be need to be done again in February.