Assuming it's a fair board, which as far as I can tell it is, then about 6.6 is the theoretical average as well. You get this by adding up the points each square gives then dividing by 22, to account for the two free roll spots. Though that doesn't take into account moving from the doubling square only being able to go to certain spots or how the doubling square and the reverse direction square combined with the starting square mean that some spots are more likely to be landed on than others for a given number of dice.