Too many variables involved for a "standard" temperature...sunny side, shady side, humidity, ambient temperature, degree of braking in the last 30 minutes, etc.; a whole host of things that can affect hub temperature readings. The accuracy and repeatability of your particular infrared thermometer also come into play; not all thermometers are created equal
What you are looking for is the "out of norm" reading. If 3 out of 4 of your bearings read, for illustrative purposes only, 135 degrees and one reads 175 degrees, then that wheel assembly
may be suspect. Could be brakes, could be tires (low tire pressure will generate more heat), could be bearings. It is the exception to your "average" reading at that moment that should elevate your attention to that particular wheel. If that wheel starts giving you consistently higher readings than the rest of them and after several readings at different times and after you've verified tire inflations, then is the time to start to figure out what is going on in that wheel; brakes or bearings...
I use an infrared device religiously, but, they are only a tool to indicate a potential problem in the making and not a spot indicator of eminent failure based on an actual temperature.
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My 2 cents, your mileage may vary...
Don
Bronwyn
2 Cats; J-Lo and Ragamuffin
2011 Keystone Cougar 318SAB
2011 Ram 2500 Longhorn CTD HO
Built in brake controller and exhaust brake
Tri Glide TrailAir Pinbox / B&W Companion Hitch