There really is no standard that I know of. It depends upon how the tub/shower will be used the most. If it will be used for showers 90% of the time, then I would put the niche in the second bay up near the shower head and at a height that is comfortable to reach when showering. If it will be used more frequently for bathing, then I would move it back a bay and lower the placement to be easily reached from a sitting position.

i'm not aware of a Standard, but you want it a bit out of spray for practical purposes, and beyond that it's probably preference--climate, whether you stand under or out of spray when shampooing, etc. prob somewhere in the range of 2-3 feet is typical, but it also depends on the scale of your shower.

I am not crazy about niche on the long wall..when looking into shower all you see is the ugly shampoo and conditioner bottles .....I am loving these luxury dispensers...they can be hung on wall directly under shower head.....I would just add a pretty Corner shelf for razor or shower sponge....add a beautiful mosaic on large back wall.

Do you have an after picture? I'm curious to see how it turned out, as I am trying to determine where to place mine. Thanks!