I started off a keys player & to me sustain is a set level the note will play for until the note is released. Bass guitar notes decay or release, depending on how you look at it. Decay being how quickly a note fades to the sustain level when played (on bass that level is 0 ) & release is how long a note lasts after being released, they don't sustain.
However, I think that when someone is referring to the length of sustain in relation to bass, they do mean the decay/release time & I accept it as that.
I would like a bass that would hold a sustained note as long as it was fretted, but I believe these are called synths