Mine usually will show like 15% or so on gauge after awhile. By the time it hits the first hash mark at 25% it will eventually go into regen soon. I'll hear the engine sound more clacky at idle and low throttle like a 2018 solid lifter and know it's going on. After its done, you'll smell it when parked. Go into park and it will kick out of regen. Not so sure if it takes off where it left off. The other day I had a pretty long one going, maybe 10-15 mins. Shut truck off because I had kids with me and crap to do. Next morning it rocked a full 20 min regen. After it was done, gauge went back to the 15% mark. Then shortly after, went to 0.
It's not the most intuitive design that's for sure. I just wish I could manual regen it. Sucks to burn up all that fuel and create all those temps to have to shut it down because life. I'd rather leave the damn thing running with the doors locked doing its regen so it can finish. God forbid you quit it too many times and it will give you the dreaded "See dealer, MPH limited"...etc BS.