I'm almost positive the SAF ENDS one frame after the hitbox comes out.
My friend is a kirby main and kept stealing the **** move (and wow is it good with kirby) and we kept getting into basically eruption duals because I couldn't get close enough to hit him most of the time. Whoever ended up releasing first would get hit, and if we released at the same time, we would both get hit, but both have super armor.
I'm pretty sure it begins, at the point that, after you release, he raises it just above his head a little bit more, and as soon as he start lowering it down into the ground is when it kicks in. This is all just based on visual experience though.