I had that happen for me on Moussorgsky's "Pictures at an Exhibition."
Studying the score, I thought some of the sections were within my reach. But after two weeks, I just put it aside.
Some long pieces -- say, 6 pages of score or more -- I will decide to play part. This is more logical if they are structured in sonata form, with sections.
So if the first 2-1/2 pages of a 6 + page piece seem to comprise a section with a kind of integrity, I may just learn that.
My time is finite, and I am just an amateur after all.
I am thinking of playing the first part of MacDowell's "Mid-Winter" as a piece to learn. The later parts of the piece have extensive, very fast runs that don't suit my skills very well.
Hope that helps.