I think it depends on the move, the music, where in the learning process I am, and what I am trying to accomplish with that practice. If say, I am freestyling to music, I just go with the flow of the music. If there is a note/lyric that is held for a bar of music, I will hold a pose for that time as well. But in a faster sequence I might just go from one into another into another, without holding. If it is a move I am just starting to learn, I just hold it as long as I can. Even if it is only a split second, lol. 🙂
It also depends what I am trying to accomplish. Sometimes I focus on combos for a practice, so I will try to do as many moves as I can think of before coming down, or will focus on specific planned combos. If I am focusing on a new move, I will just do it over and over, holding it for as long as I can each time, without worrying about transitions or combos until it is more consistant.
I have heard people say to hold a move for a slow count of 3 seconds, it's long enough for a photographer to take your picture. Longer will help build strength and stamina though.