Simplify the following text for a Grade 7 native English-speaking person.
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 .