There’s a halflife of 12 hours specified, which decreases to 6 hours if the mempool is less than half full, or 3 hours if the mempool is less than a quarter full; provided there’s been a block since the last time the minfee was bumped due to tx eviction, the minfee will decay towards 0 at that rate. So at 20sat/vb, when the minfee is raised by 1sat/vb it’ll take ~53min to decay back; at 50sat/vb, about 21 minutes; at 100sat/vb about 10 minutes; at 5sat/vb about 4 hours.