Original: Depending on the rules of the particular tournament or league, a game may have 4, 6 or 8 chukkas; 6 chukkas are most common Usually, each chukka is 7 minutes long, but some games are played in shorter chukkas.
Simplified: The game is 4, 6 or 8 periods (units of time) of 7 minutes, which are called chukkas.