Is Downtime Normal at a Job? How Much is a Normal Amount?

I am an admin/property manager on a real estate development project that is in the works. I have a ton of downtime, like, 70% of my day, and I'm constantly worried that there are other things I should be doing. I am always on top of my work and I have a great relationship with my boss...but this just doesn't feel right. Is this normal?

I'm also relatively new to the corporate world, so I don't really know how much downtime is typical. I am also confused by the fact that most of my colleagues will stay at the office until like 8 or 9 pm doing work (granted they are all a couple of pay grades above me), and I'm always out the door by 5.

Not sure if I'm on the verge of losing my job, or if I'm just efficient.

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