My previous question turned out to not ask the question I thought I was asking.
I have read that Venus began its heating-up around 700-750 million years ago.
When did the Venusian atmosphere get roughly as hot as it is now?
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According to current models and computer simulations, Venus was a water world many billion years ago but due to the close proximity to it's host star, Sun or Sol, the water was Photocatalytically split from the UV rays. This split the water molecules and it broke down into hydrogen and oxygen. Now the hydrogen is less denser than the other gases so it had an hydrodynamic escape. Left with oxygen, the highly electronegative component reacted with carbon and that lead to the formation of $CO^2$, other geological processes like volcanic activity (that occurred 2.5 million years) also may have played a role. The CO2 absorbs sunlight efficiently since it is an greenhouse gas and has high heat capacity, this lead to a runaway greenhouse effect, heating the planet drastically.
However it started to maintain this sort of thermal equilibrium when CO2 started to heat up till it matched the temperature of the Sun, applied with inverse square law. This is due to the Conversation of Etendue, which means the maximum a ray of light can heat up an object is equal to the temperature of the blackbody that emitted it. So technically it can't heat up more because it is at it's max temperature with respect to it's CO2 level. Presently, This thermal equilibrium is thought to have been started when the CO2 level matched today. The atmospheric pressure of Venus = today might have been >2.5 million years because volcanic activity started at that time, and volcanic activity leads to rapid increase in CO2, which would heat it up further. Therefore thermal equilibrium established somewhere between from 2.5 million years upto the present day
Thank you, Hope it helps you!
Note: Venus is not in the habitable zone yet it once hosted water is due to the dimmer Sun, during that period the Sun was almost 20-30% dimmer, so technically at that time the Goldilocks zone radius was much smaller, however as time passed by the Sun brightened so the water evaporated.