Too far away in terms of a human lifetime at current technology but perhaps not when thinking in geologic time. Assuming our star system travels at approximately 514,000 miles per hour around the galaxy, 100 light years is about 1,303 years away.
Probably the only reason ETs haven't gotten rid of us when they visit is, we taste bad. :^)
But virtually everything in the galaxy is orbiting in the same fashion. The relative distance to this "second earth" won't change significantly as we orbit the galactic center.
A light year is not time. It is distance. Specifically, it’s the distance light travels in one year at 186,000 miles per second. Multiply the number of seconds in a year by that number and then by 100 and you will have the distance from earth to this other planet in miles. I know. The number is truly astronomical.