"Practice" is a curious word in that it implies the existence of something else which is "the real thing". For a performer, presumably a concert or a recording is "the real thing". However, for someone like me who never performs and spends 90% of his time at the piano in improvisation, "practice" ceases to have any functional meaning - except perhaps the five minutes of technique night and morning on my silent Virgil Practice Clavier.
I also doubt that it is possible to "practise" improvisation; one either does it or does not do it. I usually play the piano for two or three hours a day, not in one session though, and it couldn't correctly be called "practice".