If I got a computer science degree today, how long would it take for what I learned to become obsolete?

My computer science degree required most of the requirements for an applied mathematics degree including probabiliy, logic, computational math I.E. the ability to solve complex series of equations, physics and all the same general university requirements as other university degrees. When I studied the computer technology was Vax server, IBM mainframe, the primary language was C. When I was certified using Microsoft server based data servers 10 years later, all the data operations learned in school were the same with a few additional operations. Most of that equipment is still in use in places. However, as part of my work, I typically had 1-4 courses a year, usually provided on the job for free. Most applications have Html based user interfaces running either on a server or personal pc. The operating systems change every 2-3 years. You learn how to learn technical aspects of computing applications as part of the CS degree.

Short answer: Your knowledge is already obsolete. Long answer: If you learned the whole idea behind how things work and why they work, those will always be useful to help you prevent going through the same trial and error that happened for many years. If you just know every single lesson you got and you remember everything they said without joining information together, then you learned nothing.

It probably won't ever be obsolete (maybe if we all use quantum computers in 40 years instead of regular ones but even then!) since most of the material taught in the computer science curriculum is a combination of theory, math, and logic. Now, the programming languages and the tools you learn may become obsolete but since you are well versed in the core concepts, you can easily pick up new languages and tools as the industry adapts! There's never been a field where it was that easy to switch tools & languages. Well trained programmers can become familiar with new languages, frameworks, etc in a matter of days!

Depending on where you got the degree, it may already be obsolete in the practical aspects. Theory in CS has held strong for some time and will likely hold for a long time - but you can never be sure about that.

What you learn never becomes obsolete. Schools mostly teach basics about computer science. By basics, I mean fundamentals, processes, algorithms etc of various languages, technologies. These are never obsolete. They evolve over a period of time sometimes called by completely different names or sometimes they get refined over a period of time. Another important point is that once you get into this field and start learning, you become aware of the things that are changing. CS is a field where you need to be in a continuous learning mode through out life, even when your school is complete and job begins. So don't worry about when things will become obsolete instead focus on getting the basics strong. They can guide you through sometimes even larger paradigm changes

In terms of software skills, just going from my own experience, I'd give it 6 or 7 years after graduation. I graduated with a BSCS in 1993. For the most part my skill set was very relevant over that period of time. The big change came in 2000, when web applications were becoming the way things were going to be developed for a while, because by then Java from Sun had started to take over, and later .Net from Microsoft. What I mainly noticed with that was that many of the CS skill areas I learned in school, like Data Structures, understanding how a C program worked, were not as relevant, because a lot of the logic I used to implement by hand, in building my own data structures, or my own database API, or my own servers, was already programmed into frameworks that I just used to get stuff done. It's not that the knowledge was completely worthless, but it was not worth as much, and I had to apply it differently than I had just a few years earlier. I had to focus more on coordinating logical processes than I used to, which wasn't bad. The main thing that chaffed me, though, was I was using frameworks more and more, and I had less freedom to create my own implementations, mainly because of the need to integrate systems in a limited period of time, and I just needed to grab and use software that would interface and translate between them. As time passed, I started feeling more like a plumber than a developer, and it was pretty clear to me that my practical skills I learned in school were becoming less and less relevant. I agree with the other answers that the theory aspects of CS stay relevant for a long time. You can use that theory to learn new technologies as you go. I'd venture to guess you'll be positioned to make better use of that technology, relative to those who don't know the theory in CS.

It depends on what you have learned with the degree and how well you had understood the details.Concepts like Data structures and Algorithms will never become obsolete. There may be new concepts but still it will be based or built on the already existing concepts. A guy who is very good at 32 bit operating systems basics wouldn't take long time to understand working of 64 bit OS. Very rarely we see an entirely new concepts. One more example is micro processors. All Intel processors' working can be understood easily with little effort by having a thorough knowledge of a simple 8086. Hence the current knowledge even if it becomes obsolete in practice you will not become clueless.  But employment it depends more on your practical skills like coding and ,apart from your learning (programming language or IDE used) becoming obsolete, without practice you will for sure loose those skills in time. In my opinion you will become unemployable soon but not clueless. All said you will be requiring just minimal effort and time to become good at coding and tools again assuming you were already good in theories and concepts.

Computer Science is about reading, writing and counting. Those won't be obsolete any time soon.