For a quick one-shot to test out the system, most of my players will just skim the rules, and that'll be enough. But if I run a long-term campaign I expect my players to have a basic grasp of the rules, but most importantly know how their characters rules works.
I will happily explain and help players get started. But if they ask me for 100th time which die to roll; don't know how their character works; or similar basic things, I will get pissed at some point and ask them to please learn the game we're playing.
You don't need to know every single rule, but a basic grasp of them is just the bare minimum or I will kick them out of the game at some point. This hasn't happened to me yet, all my players are very good at picking up new systems.
"The intelligence of that creature known as a crowd is the square root of the number of people in it."
GNU PTERRY