I usually stay hard after I bust even after a few rounds but sure even if I lose some hardness I can carry on, sometimes might have to hold myself to prevent any buckling until fully erect again.
Also even when I'm done and go flaccid as long as I'm still inside I can leave it in until I start to get erect again, once my wife is warmed up I can also just fold myself inside her while totally floppy which soon wakes up my dick again anyway.
To be fair I don't increase much in length overall between soft and hard, about an inch and a half, most of my change between soft and hard is in the thickness of my dick.