[tinco]

No, a game is only over when both sides concede, and players concede when they recognize that any move they could make either does not change the score or would cost them a point (playing in territory that is already yours effectively reduces your territory as only empty territories are counted as points (in standard japanese rules)).

If one player would never concede you would keep playing until the board is full except for the unfillable eyes of the player that wins. Obviously that would take a long time and many negative moves of the losing player.

So you stop playing when both players are confident the game is over and agree on who has won.

[pmontra]

That problem of "negative moves" is solved by the Chinese rules. A point is scored under them for each surrounded intersection plus each stone on the board. So you can defend any supposed weakness without losing points (you still lose sente and points if you needlessly defend when the score is not settled yet). You can even reduce your territories to two eyes each if you feel like.

By the way, the Chinese rules let solve neatly all those cases handled by special rules in the Japanese rules. The bent four in the corner is the most notable one. Playing it out with the Chinese rules is a neat explanation why that corner is defined to be dead: the surrounding player defends any weakness without losing points and starts the ko. The other player has no ko threats and dies. Those defensive moves lose points under the Japanese rules so they have to make a special rule for that shape and many others.

The only problem with the Chinese rules is that scoring takes longer and completely destroy the shape of the game: you fill in the territory of one player, take out the other's stones and count by grouping the stones in convenient shapes. Furthermore if you want to count during play you must remember how many stones have been captured because prisoners are returned to their bowl and are not stored in plain view (the score penalty is paid by not having those stoned on the board). Japanese rules are a shortcut that makes scoring easy but the tradeoff is the dictionary of special cases at the end of the game.