Well this gets subjective almost immediately, but in a general sense, not closely related to one instrument, I think of phrases as the music within the length of time (la dee da dee dum), as opposed to the length of time itself (1234, 1234). I wouldn't play a cymbal crash at the end of 8 bars if the musical climax didn't happen until the end of bar 9. The barlines aren't as important as the music sitting around them.