I feel like to be a classic it needs to withstand the test of time, as others have said, and have steady popularity throughout the years. Plus, some like Frankenstein are classics because of how groundbreaking it was. It was horror written by a woman in a time when women did not write anything other than poetry or romance/realistic fiction.
Gone with the Wind is one of my favorites, on a side note. It’s only a few pages longer than The Lord of the Rings, but I feel that it’s not as “easy” to get through. You got to really be dedicated to it to get through it. I had to read it twice before I finally got through it. I loved it, but it takes a lot to get through it.