Runniness seems to be in most dictionaries. I'll add it to our list.
But, common or rare? The word doesn't seem to be used often. When it is, it's usually in relation to cooking, or occasionally noses. It's a bit of a toss-up. Anybody who sees the word will know what it means, but some people may refrain from playing it because they doubt it's a real word - not every conceivable -ness extension is a recognized word. The 10-billion word News on the Web corpus has only 16 occurrences. I'll make it rare.