Not a dumb question, and yes there’s no one real “standard.” It’s just what each facility is able and willing to offer. Some places won’t charge if it is just a couple of days. Others do, and certainly all charge for longer.
I’ve seen barns that do pasture board in the sense of, a dozen horses or so out on a 20 acre pasture together, and that’s it. Not fed grain, MAYBE fed hay in winter depending on the facility/location. No stalls available if needed. I’d say this is probably the least “common” situation I have seen, because having that much pasture available for boarding barns is becoming more and more rare.
More commonly I’ve seen pasture board as a 2-5 acre field, 3-5 horses together. Almost always fed hay in winter. Hit or miss on feeding grain. Hit or miss on stalls available for emergency.
In Florida, near bigger cities like Tampa, somewhat common for places that offer pasture board (rare) is “pasture” board in small dry lots, 2-4 horses per lot. They’re out 24/7, but there’s not really grass, so board includes hay and grain just like stall board. Usually don’t have a stall available for emergencies.
When I had my Ocala barn, I offered pasture board with the choice of 60x120 dry lots, 1-2 per lot, 1-acre pasture with a group of 2-3 together, or a five-acre pasture I’d put a max of six in. I was all sand, had grass but not lush or thick with any meaningful nutritional value, so my pasture board included hay 24/7 year round, and grain for any that needed it. I did have emergency stalls available, but only because I had 2-3 of my own horses and I kept reserved stalls for my horses. Mine live out 24/7 too so assuming mine weren’t all laid up at one time, I’d let someone use my stall if needed for an injury or illness. I put in my board contract that if a stall would be needed for injury or illness, I charged something like $20/day, $100/week, and for a month or more required a new board contract for rehab board instead of pasture board. Never had a pasture boarder need it, though.