College credits almost never expire, so you don't have to worry about putting those credits to use before a certain period of time passes. However, as time passes, it may become more difficult to use your credits at other institutions or even the institution at which you earned them. In general, it pays to make use of your credits as soon as possible.
Returning to School
College credits are good indefinitely. It's true that technology changes through time and so do many courses. For example, an astronomy class or biotechnology class taught today would be radically different from the same course taught 50 years ago. Fortunately, those differences don't matter in terms of credits. However, there are other factors at play that you'll need to consider if you're looking into transferring existing credits or using partial credits to finish a degree or earn a new degree.
Some credits can become outdated. The specifics vary depending on where you're studying, but in the example above, if you had taken classes more than five or 10 years ago, it's not likely you'd be able to enroll in an advanced biotechnology course even if you had the prerequisites under your belt. As you might imagine, courses in the sciences tend to become outdated more quickly than classes in other disciplines, but ways of teaching and core knowledge in certain subject areas do change through time.
If your credits are ancient, talk with an admissions representative or academic advisor before assuming that you'll be able to use them to take new classes or earn a new degree. Your credits may not expire, but they're also probably not worth much if they're very out of date.
Let's say you earned all of your credits at one university and now want to transfer some of them to another school so that you can finish a degree there or begin a new course of study. Is it possible? Each school has a different policy. Although most are open to accepting credits earned at a different institution, that's not universally true.
Before you can successfully transfer any credits, you must have a school official evaluate your transfer credits and determine whether they're viable to use or not. In addition to sending a transcript to the new school, you may have to fill out an application and submit other materials. For outdated credits, you may even have to offer previous coursework or syllabi from the classes you took. It's also worth noting that some schools are choosy about the credits they accept. If you haven't earned your credits at an accredited institution or if you've earned them online or through some other method that is outside the mainstream, you might have more difficulty proving that they're valid and should be counted.
If your credits aren't accepted, you may need to take a collection of refresher courses. The courses will establish academic standards for the institution at which you're newly enrolled and will serve to provide you with a solid background in your chosen field of study, which can be useful if it's been a while since you've taken those last courses. In some circumstances, it can be useful to sign up for refresher courses or general education courses even if your credits are accepted, especially if your new institution demands more rigorous work than you previously had to deliver. If you can take a placement test or undergo an evaluation before you sign up for new classes, take that opportunity to gauge how prepared you might be for continued study.
No Sell-by Date
The idea behind college credits is that the knowledge you gain is useful no matter what. Even though cutting-edge technology and research is constantly changing, the value of your education is worth the same now as it will be in a century, so there's no "sell-by date" on college credits. However, if you do plan to transfer or use existing credits for a new degree, it's to your advantage to do it quickly while that knowledge is still fresh in your mind.