I think it really could depend. Yes, there's a strong possibility that you get excessive cutaways to appendix content and a lot of invented side-drama to pad it out, but it's also conceivable that a longer take could include much more of the original dialogue. As a very silly and trite example, take Eowyn's iconic "I am no man" line from ROTK. In the book, that line is an entire speech, and it's fabulous:
> “But no living man am I! You look upon a woman. Éowyn I am, Éomund’s daughter. You stand between me and my lord and kin. Begone, if you be not deathless! For living or dark undead, I will smite you, if you touch him.”
The Mary Sue did a great long-form piece on how unfortunate it was that Eowyn's character wasn't managed better: https://www.themarysue.com/the-story-of-eowyn/
In any case, this is just one small example, but it's easy to imagine that with a longer treatment of Bilbo's birthday party, the hobbits might have been able to be more fleshed out as well, and less of just dumb comic relief. That said, even the 1981 BBC radio play that's considered by many purists to be the gold standard as far as LOTR dramatizations is only about 13h, so perhaps I'm being pretty optimistic to imagine that doubling that runtime could go anywhere good.