I usually regard my libido as riding high when I'm noticing I'm prone to involuntary sexual fantasy and erections. I might feel "pent up." However, these two things are the whole of my consciousness of it, no level of introspection can really determine how charged the libido is.
At the point where you're noticing some kind of uncomfortableness from your sex drive, most psychologists would recommend seeking release. If you've got a receptive girlfriend, that's fabulous, but prone to serious possible consequences. Masturbation gets the job done with minimal negative societal fallout, provided you conduct yourself discretely. Hint. [http://laurenoutloud.com/main/wp-content/uploads/2010/11/u_of_t_masturbating.jpg] Either way, after release, the libido will lower and you're free to move on with your life relatively free of mental distraction until it recharges.
How often will this happen for you? More when you're younger, but it's really tricky, because there's a definite difference in different people's libidos. Some can repress it without much consequence, others find that extremely uncomfortable. The important thing is to not let your libido push you into doing something stupid you'll regret. Just because you're mildly addled by the animal within doesn't mean you get a free pass not to use your brain.