Natural for genuine native speakers.
Yes, a native speaker who hasn't used the language in a long time might hesitate in borderline cases but the basic distinction is as natural as 'the' and 'a/an' is for English speakers (which non-natives need to learn rules for, rules which don't always work).
For native speakers the language doesn't describe reality, it _is_ reality, for Rich Polish is obviously a foreign system the rules of which seem arbitrarily and needlessly complex while English is his reality. No non-native speaker attains smooth fluency without some kind of formal study as many fossilized and non-idiomatic forms clog up the flow.