Tailored Cover Letter?

Hello everyone!

I am looking for an IBD summer internship. I keep reading posts about how cover letters should be tailored to every position you are applying for and how generic cover letters are automatically dismissed. However, I find it hard to believe that people who apply to 30-50 banks and boutiques actually take the time to research every single one of them and change their cover letter accordingly. This seems excessively time-consuming.

My question is: how much should you adapt your cover letter to every specific company vs. how much of it should be more "generic"?

Edit: Just to be clear, I am European so I am mainly applying to London internships.