When did we figure out that Venus was too hot for humans?

In your title you address habitability, but your question refers only to temperature, so I'm not sure which you're asking about. This answer focuses on habitability in general (although it includes temperature). That is why I am citing Venera 9, which landed on the surface, and dispelled certain doubts about Mariner's measurements, as well as adding information about factors theretofore not known or measured. (Question was subsequently edited to refer only to heat, so most of the discussion here becomes irrelevant to the question as posed, but does contribute something IMO to the question of 'Life on Venus' and the history of our exploration of that planet.)

I think it's safe to say that the first time we acquired what appears to be conclusive evidence concerning Venus's inhabitability was in 1975 was when the Russians orbited and landed on Venus for the first time with their Venera 9 mission:

Venera 9 - the orbiter was the first spacecraft to orbit Venus, while the lander was the first to return images from the surface of another planet

Here are some of the things we learned from Venera 9:

Supra:

Venera 9 measured clouds that were 30–40 km thick with bases at 30–35 km altitude. It also measured atmospheric chemicals including hydrochloric acid, hydrofluoric acid, bromine, and iodine. Other measurements included surface pressure of about 90 atmospheres (9 MPa), temperature of 485 °C, and surface light levels comparable to those at Earth mid-latitudes on a cloudy summer day. Venera 9 was the first probe to send back black and white television pictures from the Venusian surface showing shadows, no apparent dust in the air, and a variety of 30 to 40 cm rocks which were not eroded.

Let's see:

• Atmospheric chemicals such as hydrochloric acid, hydrofluoric acid, bromine, and iodine:

  Certainly not life friendly material.

• Surface pressure of about 90 atmospheres:

  Such pressures would be a virtually insurmountable challenge to life as we know it.

• Temperature of 485 °C:

  That kind of heat would decompose and vaporize any of the chemical compounds necessary for life .

• No apparent dust in the air, and a variety of 30 to 40 cm rocks which were not eroded:

  This would indicate no precipitation and an atmosphere that is perhaps dense and stagnant. Life as we know it needs water from rain and something circulating to breath

But, considering what we know today about terrestrial extremophiles, one could argue that perhaps in spite of all these apparent obstacles to life, life could still theoretically exist on Venus. Also, perhaps Venera landed in a very exceptional location, as has been mentioned in the comments.

In addition there are those who believe that in much earlier times, Venus might have been more hospitable to life. See: Studies have suggested that billions of years ago, the Venusian atmosphere was much more like Earth's than it is now, perhaps not unlike the depictions of Venus in fiction that you mentioned, and it is conceivable that life arose on Venus then and proceeded to move above or below the planet's surface as conditions there become inhospitable. See: Although the surface conditions on the planet are no longer hospitable to any Earthlike life that may have formed prior to this event, the possibility that a habitable niche still exists in the lower and middle cloud layers of Venus cannot yet be excluded

Conclusion:

According to our knowledge of conventional earthly life forms, based on data from Venera 9 we first learned that the surface of Venus is indeed uninhabitable in all likelihood.

Nonetheless, even assuming Venera landed in a typical Venusian location, and excluding the possibility of previous surface life that migrated, we still have no conclusive proof that the entirety of Venus is uninhabitable for any and all forms of Life. We don't even know about all the niches where life might exist on Earth - new habitats and novel forms of life are discovered here not infrequently, such as the extremophile forms mentioned above. As our knowledge of earth expands, it's becoming increasingly difficult to say with certainty that any particular celestial body is uninhabitable.