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Jigsaw puzzles with an extra large number of pieces aren't as hard as they sound, because they're divided into sections with each section in a separate bag, so that you can solve each section independently and join the sections together at the end.
Of course, to a real jigsaw puzzler, that wouldn't be a true challenge at all, so I mix every bag together before starting.
I started this 13224-piece puzzle 821 days ago and finished today. It doesn't quite fit in my room, but I'm planning to slide it out the door in pieces and reassemble it before attaching it to a wall.
It actually would have only taken 707 days, if it weren't for the fact that over 400 days in, a cat I was looking after mistook a boxful of dark pieces for a litterbox, leaving it unsalvageable. I bought a second copy, hoping to just be able to swap the dark pieces in and out, only to discover all puzzles are cut differently.
In the end I had to start from scratch, but kept the bags separated until I had solved only the pieces I had solved originally (in a quarter of the time), before fully remixing the rest of the pieces.