Acid lasts a long time, 8-12 hours probably for a single average dose. When you start taking more it can increase to 12-15 hours or even more. However, only the first part of that is the peak, and after the peak, unless you totally freaked out, psychedelics (including LSD) tend to get much easier to handle and for me, they create a state of sublime bliss.
Just remember to go with it, and the come-up is the most likely to cause you anxiety, so if you start to feel bad, just remember that it WILL end, and that it will subside once the drug fully comes up, as long as you don't give in completely to the anxiety. And also remember that acid and other psychedelics are much more than recreational highs. You'll have fun, but a psychedelic has the power to show you things that can totally shatter your conception of reality. It may not happen, but be ready for that, and make sure you are willing to have your outlook on life and the universe changed, because once you do it, you can't go back. I'm eternally thankful that it happened to me, but if you're content with your little world right now, you may want to seriously consider this.