Li-Ion batteries don't like to be drained completely - although it happens.
On my Model 3, I've gone from -2% (yes there is a reserve) to 80ish% in 45 min at initial 250KW/h supercharger. Since Rivian is roughly 200kW/h so maybe bit longer but not significant amount I would guess.
The rate of charge changes as a function of % charged so it declines as the charge gets fuller.