…load should be based on the duty cycle. So your "in cut" time should be part of defining the acceptable load. Most of the VMC spindle motors will state the load rating. An example is on a CNC lathe the spindle motor may be rated at 7.5 hp 100% duty cycle continuous… and 10 hp 100% for a 30 minute duty cycle.
Most cutter loads vary a lot on the load they pull, so it can be of advantage at times to program metal removal (assuming you’re trying to get the most out of your machine time) loads to balance out the hp needs, allowing the motors time to dissipate heat and still get maximized usage.
But I’m like you… I tend to keep my loads conservative. I like to see less than 60 or 65% loads 80% on the time on a VMC. Less wear and tear on the machine, I believe, no matter what the machine builder says. Most of us machine owners are not using our machines as throwaways in the 5th year. So we sacrifice an hour or so a day of productivity for a longer machine life and hopefully less maintenance cost.
The machine is the best employee you can get. It will do exactly what you tell it over and over. So as long as you treat it right, and tell it right it will never make a mistake, hardly ever is sick, and never comes in with a hangover.