How much time and effort do you put into your CS, Engineering, Science, and Math studies?

[AccCreate]

25 hours a week outside of class for Honors Linear Algebra! That’s brutal. This is because you were aiming for an A?

To be fair, there were some courses in which for certain sections of exams, I scored the highest in the class.
And I know some friends in which in an average exam score of a 30 (with an A being 32), scored a 100.
It really depends on your goals.
For me, grades were largely unimportant. I just simply wanted to challenge myself (and not feel much regrets after).
Nevertheless, I don’t think it’s the norm to expect such outcomes in college. College isn’t high school.

There is less return on investment every extra hour you put in. So I plan to study enough to maintain a 3.0 GPA.

Study enough to feel comfortable about the subject. That might mean 0 minutes a year (and not even bothering to attend class). Or that might mean all your free time.
And if you want to maintain a 3.0, aim for a 3.3. People tend to gain results lower than they expect.

I don’t think its worth spending an extra year or two in school, taking one less class per semester, to achieve a 3.5+ GPA for me personally.

You got all the time in the world during college. Some of my peers took 20+ credits a semester with a near full time job while maintaining a near 4.0 GPA.
Not saying you should expect a 3.5+ GPA but don’t think of it as being a trade off for “one less class per semester”.
The goal of attending college is to challenge yourself and hopefully “better yourself” within the 4 years. If your goal of college is to party and have fun instead of being a student, then college could end up being a poor investment vehicle for you.
Remember, once you leave to the real world, you are now competing against the market. A market which includes people from all over the world and a market in which some group of people are willing to work more than you for a lower pay. (e.g.: outsourcing)

How much time do you think the"average" student at regular university needs to dedicate outside of class to get an B grade for classes such as Calc I, II, Discret math, differential equations I appreciate the reply!

Differential Equations is going to requite significantly more hours than any of the others: Calc I, II, Discrete math.
It also depends on your computational ability.
For Calc I, II, Discrete Math, I would assume like a few hour a week for most schools? Could be good enough to even justify an A.
For Differential Equations, I would assume whatever number of hours you spend in Calc II times 2.5~3 a week? Could be good to even justify an A.
It really depends how fast you grasp the concepts. If you could grasp the notion of Integration within a few minutes, then you probably understood the entirety of Calculus I and Calculus II right there. If not, oh well.

Are internships equally as important as a Solid GPA?

Internships trump GPA in the work force.
Especially once GPA is around 3.3.
Now, getting your first internship with a low GPA is a difficult one. How would companies judge whether to take a risk or not outside the GPA metric in the start?