The onelin’s comment is correct. Most games especially at the professional level will end in surrender (a seasoned player is counting the number of points she holds or could capture, several moves in advance).

The rule of thumb is, pass your turn if you can no longer make a move that works to your advantage (ie. don’t keep filling up your territory if it’s secure). If both players pass a turn in succession the game is over. Count the points you control and the enemy pieces you’ve captured. The player with the most points is the victor.