The number of credits required for graduation depends on your degree program (your major). For Civil Engineers, this is 128 credits, which I believe is pretty standard. Simple math for 8 semesters gives you 16 credits per semester, which is the norm here at RPI. 16 credits is most commonly achieved with four 4-credit courses.
At most schools, a credit (or credit-hour) is directly the number of hours of class per week. At RPI, this can sometimes vary, but we’ll say that it’s the same for now. So, a 16-credit semester would leave you with 16 hours of class in that week. Think of your high school courses… about 40 hours of school a week, probably. So, you spend a lot less time in class. If you manage your time outside of class wisely, you should have plenty of time to be social. See how I made that bold and italic? It’s very important.
If you are confident with your abilities in that AP class, I would recommend that you accept the credit. For instance, if you take BC Calculus and get a 5 on the AP exam with a AB sub score of 5, you’re probably safe to waive out of at least Calc I at RPI. If you take AP Physics B and get a 4 on the exam, you may still want to take Physics I, even though you could get automatic credit at RPI if you wanted. Some people decide to waive receiving credit to reinforce the material. In short, if you had a good AP teacher and know the material well, I would recommend that you accept the credit.