If its your first time, and its real LSD one hit will open your mind and make you laugh for hours. Have fun, and remember that once you have peaked and you start to wonder if you are supposed to feel this way or if it becomes overwhelming just remember your on ACID dude, its designed to open pathways in the mind that are never opened. And you will come down no matter how long you feel it.
My first acid trip lasted 15 hrs. I was spun the whole next day... Never thought I'd feel the same again. Well, I came back and so does everyone else...
Have fun, be smart, know your source!