Actually high quality acoustic pianos often do have very long sustain because they have wonderful soundboards. I have a top quality grand piano and I once checked in excellent acoustic conditions and and could still physically hear a sustained chord after about 30 secs which would typically be a lot more than 8 beats. Number of beats in 30 secs depends on the tempo so at 100 bpm for example, 8 beats is only about 5 secs. Even in a slow piece at about 60 bpm 8 beats is still only 8 secs.
But even the best digitals don't come close to this. And one of my few criticisms of Yamahas digital pianos in general, and arranger pianos in particular, is that the sustain is not great and dies way too fast. There didn't seem to be much I could do it make it better.
Mike