it is a matter of eyesight quality first, but then, you need some training as to avoid focusing to something too close as NEEMA said. A trained pilot with good eyesight can see quite far but it is hard to get a measurement (you’d need to know the exact position of the aircraft you see).

Once, at the Ferté Alais airshow I spotted Alphajets of the Patrouille de France on their last turn inbound over Orly (25km away, which is some 13.5 nautical miles (15.5 statute miles).. without smoke of course. At altitude, the air is much clearer (less haze, pollution and so on), so, if you have good eyesight and are used to look “far” (again), and the glass you look through is of good quality as well, I’d say that for an aircraft of the size of an airliner, you can spot it at distances of more than 20 miles