Light travels about one foot per nanosecond. One nanosecond is the period of anything vibrating at 1GHz. If you're in a typical room in a house or office building, you're looking at things maybe ten, twenty, orthirty feet away. What you see is how things were 10, 20 or 30 nanoseconds ago. For the everyday things in an typical Human's life, this is so small it hardly matters. Only physicists and radio engineers care.
What about the delay due to air? Vacuum is by definition a "medium" with an index of refraction 1.00000 while air, at the surface of Earth, at a comfortable temperaturn and normal pressure and density, aka "STP", has an index of refraction about 1.00029, for visible light.
Out of a travel time or 10, 20 or 30 nanoseconds, the slowdown caused by light is a fraction 0.00029 of that, which is 0.0029, 0.0058 or 0.0087 nanoseconds.
What about when you're outdoors looking at mountains ten miles or so away? The regular speed of light means a delay of about 52000 nanoseconds, or 52 microseconds. The effect of air, compared to vacuum, amounts to 0.00029 of that, about 15 nanoseconds.
The exact index of refraction of air depends on temerature, density and humidity. The NIST has a page detailing this variation.
It should be noted that this is only a time delay. Any repeating activity will appear to occur at the same speed. If a light is blinking every 1.000000000 seconds, you'll see each blink several nanoseconds later, but still ocurring every 1.000000000 seconds.
When looking at the Moon, Jupiter, stars, or anything outside Earth's atmosphere, you're looking through vacuum mostly, but there are several miles' worth of air between you on the ground and empty space. Our atmosphere tapers off gradually. If you brought down all the thin air in the upper fringes and packed it down so it's all at STP, I think it comes outto be about 10 to 15 miles thick. Well, that's about the same as the mountain example I just gave. So you would see a supernova go off about 15 to 20 nanoseconds later due to Earth's atmosphere, compared to if the air weren't there.
Fun fact: radio astronomers can measure the delay of microwave signals from pulsars due to the interstellar medium. This medium is what most normal humans call "vacuum" but it's not perfectly vacuum. The index of refraction is tiny, and depends strongly on frequency. Measurement of this variation tells astrophysicists something about the few neutral atoms, H2 molecules, and free electrons and protons, drifting around between the stars. It's mostly hydrogen molecules, at about one million per cubic cm.