I usually keep them short-ish (max 2mm, seldom 3mm). They are very weak and can't grow longer. I would prefer them longer but anyway they are too cumbersome if they get too long.
edit: besides, with the things I do (mountain biking, sometimes fixing my bike on the dirt track, wearing latex gloves in the lab pretty much 6-7 hours a day) it's pretty much impossible to have long nails.