This matches my experience, and I've been 'nervously' anticipating this year's Advent of Code. I managed to keep to the spirit last year and get everything done by Christmas day (though admittedly with some days bleeding over into other days due to pesky family/other commitments), but even this relied on having the last week or so of the month be relatively free for me.

While I've usually been able to do the first half of the month's puzzles in the day before breakfast, over lunch and in the evening, the increasing difficulty does mean that later puzzles can really eat into a day, particularly if you happen to go down a bad path for your solution.