I don't have a niche, but I have corner shelves that are 51" off the floor, if that helps.

We have two niches, a smaller one that is on our pony wall and therefore lower, and a taller one with two shelves up higher. I can measure their distance from the floor when I get home. When we decided where to put them, my husband and I held up some of my product bottles to decide where they'd be most convenient and then we adjusted +/- an inch or so as needed to line them up with a row of tile. Reaching up to get a bottle of shampoo isn't an issue but we made sure that the shelf for my oversize pump bottles was at a height that made them easy to use.

It's all personal preference but the best height, IMO, is whatever lines up with the tile AND is out of the splash zone as much as possible.

You can place them as high or low as you like, so long as you're comfortable with them.