Maximum Length Game

The game begins with a row of 42 coins, all showing 'Heads' up.

Each player, in their turn, chooses one of the coins showing 'Heads' up - flips the chosen coin, and also the coin immediately to its right (if one exists).

If you manage to make all the coins show 'Tails' up on your turn, you win the game!

At most, after how many turns will the game end?