How hot is “hot” on wheel bearings?

That's a really good question!  Many trailer folks worry about that without understanding what "hot" is.  The answer to that depends upon the source of the heat. First, a wheel bearing should never be so hot that you can't put your hand against the hub face, or grease cap. It's probable that you have a stamped hubcap in the center of the wheel that prevents you from even accessing the hub directly. That larg-ish cap is part of the wheel assembly, not the hub itself.  The bearing hub is cast iron (steel) with a small stamped steel cap on the end. Assuming you can't actually feel the hub, put your hand on the wheel face in the center. If you can hold your hand on it, it's okay. If it uncomfortably warm to the touch at the wheel, the hub is too hot. 
90% of heat induced failures in hubs comes from hot brakes, not bearings.  Keep in mind that the metal parts of a bearing set would be perfectly happy and function just fine at temperatures that would sear the skin off your hand.  The. Problem (and answer to your question) lies in the boiling point of the grease within the hub. Once that happens, the pressure causes the wheel seal to fail and quickly empty the hub of its lubricant.  I have seen that many times. 
What to do with a hot hub?
1. Take a break. Get off the road and let the hub cool down for 15 minutes or so.  
2. Dial back your brake controller. That's probably where the heat originates...not the bearings.
3. Check behind the wheel to see if you can see grease around the brake drum that has escaped the hub. (It will probably be obvious, as in, a sloppy ring of grease thrown off the drum covering the back of the tire, the frame, the wheel etc.)
If the hub cools down quickly, there's no grease-ring on the back and the wheel feels tight when pushing/pulling on the top of the tire, you can try to move along cautiously for a bit. After a few miles, check it again. 
When wheel bearings fail mechanically (something actually breaks inside the bearing) it happens rapidly and catastrophically, almost always resulting in what's called a "spun bearing" where the bearing race itself starts to spin against the axle. That generates heat very quickly (within a mile or less) up to the melting point of metal with lots of smoke and noise.  Thankfully, that seldom happens. The usual scenario is you are coming down Wolf Creek Pass with your foot on the brake. The brakes heat up the grease to the point that it loses viscosity, boils and blows the wheel seal. Once the grease escapes, things mechanical go south quickly. 
OR:
You have a brake "dragging"...as in, the adjustment is too tight or a brake shoe is "stuck" and doesn't retract away from the drum when the brakes are released. Different scenario, same result. 
The answer to your question lies in the boiling point of the grease.  If you can comfortably lay your hand on the hub/wheel face. You're fine.  I check my hubs every time I go out.  Drive a few miles, pull over and feel the hubs.  If they pass the "touch" test, check them each time you pull in for fuel.  It's also common (on my trailer at least) for one side to be warmer than the other. As long as they aren't wildly different, they're probably fine.  
Tapered (wheel) bearings are rugged and can take a lot of abuse. What they can't stand for long is a loss of grease. Do the "feel" test, watch for thrown grease and when in doubt, back off on the brake controller a bit...

WilliamA