As another recent newbie, I'd say that if you have the time, money, and inclination, definitely sign up for both days. By the time you drive to the track, set up your pits, and get lodging (if required), you may as well maximize your time on the track and do the whole weekend.
You'll likely get a little over two hours of total riding time per day, so that's somewhere in the range of four-five hours total riding time for both days.
Eric is correct. If you can swing it, you'll learn a ton more in two days than in one. It will give you a chance to take your time, get aclimated, and get to know the crew. And use the coaches. They will help you as much as you ask for it.
For the most part people sign up for a weekend and ride both days. There will be 7 20 minute sessions per day for each group. It doesn't sound like much but you'll be exhausted end of day one and sore end of day 2. This is a sport just like any other and repetition and consistency will help you. We all started right where you're at so keep asking questions and when your at the track, introduce yourself to your neighbors everyone is very friendly and willing to help, especially those new to the track.
You will no doubt love PittRace! You'll definitely want 2 days there, as one of the longest tracks in the country, it'll take you damn near all of the first day to memorize which direction the next corner goes!
Most excellent! Pitt is the favorite of many here. At your first day's riders meeting let the coaches know this is your first day ever at a track. We'll assign you a coach for your first session (or 2) to break the ice and get you familiar with the track. I guarantee that by the end of the day the hook will be set and you'll be happy you signed up for both days.