My sister and I had our fifteen minutes of fame. It wasn't good. We were blindsided by strangers looking us up on FB and sending actual hate mail to us, so we locked everything down. This was back in 2007. I haven't had a single problem since then.
I have it set so you have to be a friend of a friend to search for me, but you can set it so nobody can search for you unless they're already your friends. What's really fun is when two people have those settings, they know each other outside of FB, but don't have any mutual friends. Neither can search for the other, aside from coordinating a time where they'll both be on, one will change their settings, let the other know, the second will send off a friend request, and the first person will lock their settings back down. BTDT. It was actually kind of fun.
As to how much time you'll spend, it's up to you. I personally hate all the games on there. I've blocked all the app requests I've gotten, and keep blocking them as they come out with new ones. If you don't have very many friends on there and don't care to use that to follow their every move and instead use it as more of a chat service, it shouldn't be too much time at all. I'm usually only on FB for at most half an hour a day, and that's only five minutes here or there checking in on things. Tumblr, on the other hand, has been like eHell and sucked away hours of my day every day.