Switchgear itself is designed to NOT have heat losses if at all possible since heat losses mean resistance which means poor connections or exceeded capacity. Overall losses will vary depending on the number of transformers and other sensing devices, plus the loading of the switchgear and bus resistance so I doubt that there is a "rule of thumb" that would be meaningful. I would be surprised however if it was over 1% aggregate. Losses through Vacuum Contactors for instance are shown as 0.03% (99.97% efficient)
Drives are another story all together. Any decent drive manufacturer should be able to tell you the desgned heat losses at variuos load points in the speed curve. They need that info in order to design their own internal cooling systems. A worst case scenario (assuming new or nearly new drive designs) would be 5% losses at full speed, and as much as 10% losses at 1/2 speed. This is typically true for MV and LV drives. Some (if not most) would be better than that. Drive manufacturers will often list that info as "system efficiency".
If your motors are in the same environment, keep in mind that motors on VFDs run hotter than normal as well. If you look for a "Throughput Efficincy" rating of the drive and motor as a system, the losses are all heat as far as the building AC is concerned. Subvert the dominant paradigm... Think first, then act!
