It really depends on where you are and how seriously they take their interns. I've seen interns accomplish a fair amount in 3 months where I work (Mozilla). As a concrete example, we had an intern propose and implement a new devtools feature that lets you see what your site design looks like to users with various visual impairments.
Still depending on where you are and what you have already done, some things that one can learn from an internship that one might not encounter otherwise during college:
* Working with a large codebase.
* Working with a distributed team.
* Code reviews, both as the recipient and actually doing them yourself.
* Dealing with large existing suites and writing your own tests, then dealing with the intermittent failures.
* Finding out what various people in the industry are working on.
* Finding out what jobs actually look like in practice on a day-to-day basis.
It's possible to pick some of these up via various open-source project involvement, of course, even without doing an internship.
As a personal anecdote, during the one internship I did I learned a lot about the concept of "just because it's a spec doesn't mean you should implement it", ended up fixing a ship-stopping bug for the product I was working on, made some friendships that continue until today (nearly 20 years later), learned some hard lessons about the failures of corporate IT provisioning in a large corporation, plus some of the things I listed above.
