Many players report that if you just beeline the main scenario quest only, you're looking at about 40-60 hours just to beat the base game, A Realm Reborn. Each expansion adds about 40-50 hours of content in story. You're looking at anywhere from 200-310 hours for story alone as of patch 6.5.
Tackling side content will potentially add dozens if not hundreds of hours to your game time. If you want to stop doing MSQ and make a relic weapon, level up crafters, or become an ocean fishing expert, be prepared for a lengthy grind.
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