It’s not being done outside of science fiction right now, and won’t be in the near future. “A larger scale” is an understatement - the number of cells that would have to be changed would be extremely high. And even when dealing with embryos with current technology, the success rate for actually changing genetic information is far below 100%.
On top of which, genes code for proteins, which then have some effect, like making an eye color. So if you change the DNA, the cell’s old proteins would have to be destroyed and replaced by the proteins coded for by the new DNA, then the new proteins would have to catalyze the new chemical reactions, etc, etc. How long that would take would vary significantly depending on the trait. I don’t know a lot about the molecular mechanisms of eye color determination, but I’d suspect even if you changed the DNA, it’d be tough to effect a rapid change in color. But I could be wrong.