Typically it's 4hrs @ 90. the 6hrs comes into play when we have to assemble hats, bleed, etc.
Four hours at $90 CHARGE is reasonable to make sure the job is done perfectly well, but I certainly don't think your techs are really taking four hours. That is reasonable at $90 per corner.
Disassembly and reassembly of hats is indeed tedious, so if that's what you mean by taking that long okay, and I take any meanness I projected back. But, if at $100/hour, 6 hours for a rotor/pad changeout and a brake bleed (without hats) seems excessive.