User blog comment:IamJakuhoRaikoben/Chapter 335: The Time of Life, Review./@comment-1535822-20130614204951/@comment-7418318-20130614210724

Just a little calculation: If you take 1 million people and give each one of them one minute back, you'll end up with 1'000'000 minutes = 16'666.67 hours = 694.44 days = 1.9 years.

Since Magnolia alone has alreay 60'000 inhabitants and Fiore is relatively small on the map we can assume they got several tens of million Humans on that planet.

That alone is something that would go beyond a Human livespan, but she didn't just reverse the time for Humans, but also for all other beings, plants and inanimate objects, which puts the task beyond the monumental.