When does file size become inefficient?

I am using a JavaScript file to create Dynamic Content, so the user never has to reload the page, due to the content being very simple. Mostly text and images.

So far it is all in the one JavaScript file, and I have finished the first couple of pages out of a dozen or more, and it is already at 500 lines of code, approx. 20,000 words.

My question is, does it get to a point where having a long file, so over a certain number of lines, where efficiency starts to drop. Is there any disadvantages to having all your code for content switching on one file, is there any advantages to splitting it into multiple files other then readability?

Thanks for answering a junior programmers question.