Between four and eight hours — and the picture moves that number more than the piece count does.
The honest answer is between two and ten hours, and anybody who gives you a single number is guessing. But the spread is not random, and the things that move it are predictable enough to plan around.
For an adult working alone, on a picture with detail throughout, at a steady pace and without interruption:
Notice that it is not linear. Doubling the pieces slightly more than doubles the time, because the work is not really placing pieces — it is searching for the right one, and the pile you are searching gets bigger at the same time as the number of searches does.
Online is faster, and the reason is mundane: no setup, no lost pieces, no hunting for a flat surface, and the pile is never obscured by other pieces. Against that, you lose the ability to feel a piece's shape with your fingers, which more experienced puzzlers rely on more than they realise. On balance expect a digital puzzle to take roughly two-thirds as long as the same picture in a box.
Every figure on this page is reasoning from how puzzles work, not measurement, and it would be dishonest to present it otherwise. Published estimates elsewhere are the same, though they rarely say so.
The puzzle of the day is a step toward fixing that: the same picture, the same cut and the same piece count for everyone in the world, with times verified by the server rather than taken from the browser. Once it has run long enough to mean something, this page will be replaced with numbers instead of estimates.