I would think, first, on how is it mounted. Did you use Star washers? Pyramid washers? A filler between the inner and outer lid? A hose liner in the gasket under the trunk lid? All of those things would affect the pressure distribution over the trunk and outer trunk lid. So I would guess weight is something between none and ten pounds.
And then consider how it is used. Some never touch theirs, others load it up and use it as a handle to slam the trunk. Which means the chances of damage to the trunk lid ranges between nil and certain.
Worst case - Put twenty pounds on it, then slam the trunk lid down really hard using the rack as a handle, take off across some washboard road at a high rate of speed, them jump on the internet and say it was faulty.
I carry about ten pounds max, have had one on an 1100, 1200, 1500, and over three years on the 1800, with no damage to any trunk lid. But I don't use it as a handle, either. Your experience will likely differ.
The question should be how much you can carry on the rack safely. The trunk is limited to 20 lbs, subtract from that the weight of the rack and the load. In addition, the position of the rack/load (high and behind the rear wheel) is not the most stable .
I have overloaded mine twice before we bought
our BushTec Trailer. I use it as a handle to close
the trunk all the time. I just noticed this past week-end
some little swirrly cracks around one of the legs. I think
this is because fo what IR Harry said:
If you take apart your trunk lid, (I did to install a 12volt
outlet for charging a cell phone), you can see it is hollow.
I wish I had some little somethings to fill those areas,
I am just not motivated enough to think of something,
and I don't want to take the lid apart again.
In conclusion, I'd say don't put more than 5lbs on it, and
just let the trunk shut on its own.
Weight is definitely one of the factors with a rack. Many forget about the driving condition being a factor. Put a little weight on that rack. Start hitting some road turbulence (pot holes etc.) and you get a cracked trunk lid.
I don't putt anything on my rack now. If the rack was not already on my bike, I would not have ever had one installed.
One mans good taste is another mans idea of poor taste.
What ever floats the boat of the guy that is going to have to live with it.