How many hours does it take to complete a Pokémon game’s Pokédex?

How to get close to this number

Multiply rather than guess. A modern Pokédex holds several hundred entries, and filling each one takes anything from a few minutes of catching to a long stretch of hunting for something rare or version-exclusive. Even a modest average per entry, multiplied across the whole list, produces a total in the hundreds of hours. Decomposition beats intuition whenever a total is the sum of many small repeated tasks.

The gap the explanation points at is the interesting part, and it is what makes people guess low: the main story is a small fraction of the completion time. Estimating from "how long is this game" gives an answer roughly an order of magnitude short, because completion is a different activity built on trading, breeding and grinding rather than on progressing.

Treat the figure as a rough community consensus rather than a specification. It varies enormously by game and generation, depends on whether trading with other players is counted as available, and is self-reported by the kind of player who finishes a Pokédex and then logs it. Those are exactly the conditions under which a published average is useful for scale and unreliable to the decimal.