Depends on how much fun I'm having. Honestly, isn't that all that matters?
100%, side quests, achievements etc , they're all just extra things to do in a game, other than meeting the main goal.
Going thru a perfect example of this right now playing Rayman Origins. The choices I have for each stage are: just reach the end, find all the hidden stuff, get the medal, and/or get the time trial trophy. Sometimes I go for everything, but currently it's more fun just to play through. If I have time later (unlikely), I can always come back and do more.
Oh and achievements are annoying. Most of the games are impossible for me to "complete" because they require multiplayer and I refuse to pay for an Xbox gold subscription.