This got me curious, based from NBA Advance Stats:
Rondo: 33.6min/game played
7.7min Time of Possessions per game (462 secs)
Boston: 96.9 possessions/game
33.6mins / 48mins = 0.7 x 96.9(possession/game) = 67.83 possessions per game played
462 secs / 67.83 = 6secs / possession
For conversation piece, assuming half of them are fastbreaks or touch possessions only
it will still be 12secs / possession