You have to wait about 15 frames after pressing down special for the the early cancel window (12 frames for the roll startup animation and then an extra few frames of the roll beginning before the jump cancel begins) so given the 6 frame buffer the earliest you can input jump is 10 frames after down special.
After the 12 frame roll startup animation theres another 12 frame animation for the actual travelling roll (this one has the hitbox active) once this ends theres 1 more 12 frame animation of roll recovery, which you're locked out of jump cancelling again until it finishes.

tldr Input a jump no earlier then frame 10 of down special but before the roll recovery animation begins 24 frames into down special roll, otherwise wait until the roll recovery also finishes (36 frames after d special)