Well, obviously it depends on your definition of 'beat'. Or 'finish'. Or 'complete'.
It took us 22 hours to see credits. However, hardcore series veterans and hardened Poké Trainers likely wouldn't consider the game 'beaten' until they've gotten themselves a complete Hisui Pokédex with all Pokémon caught — it was 70-ish hours before we had seen and caught absolutely everything in the game.
With that last figure, it's worth bearing in mind that we were playing for review, so that includes downtime as we took notes, made cups of tea, looked for nice screen cap opportunities, and generally poked and prodded at the world in a way you might not though 'normal' play.