This really is a multiple choice question, since it depends on the game and the situation.
Under optimal conditions (I own the game, it's a good game) I tend to go for near 100% completion. I'm not stuck on every sidequest, every secret and every bonus, but I will endavour to experience all of the game's content (alternative endings, branching storylines, etc.) and will try out most of the game mechanics (most of the character classes, weapons, etc.). I will, however, usually skip the various extra "challenges" such as time trials which just recycle old content.
If it's a good rental game, I'll try to accomplish the same, however I might fall a bit short due to time constraints, but I'll still prioritize content. If it's a bad rental game I'll give up as soon as I get bored.
The last option, bad game that I bought, just doesn't happen. Go informed consumerism!
