Earlier than most people think. Opinion forming starts before physical arrival: the booking process, the clarity of directions received, the confirmation message, even how easy the place was to find online all seed expectations that the doorstep then confirms or breaks.
At the location itself, the entrance does heavy lifting because it is the first physical evidence: approach, signage, cleanliness and ease of finding the way in are processed in seconds. But most people would say the impression truly crystallizes at the first human interaction, whoever meets the eye first, whether that is reception, security, or a passerby who helps or does not.
So the honest answer is that it is a chain, and the impression is only as strong as the weakest early link. Organizations tend to over-invest in the lobby and under-invest in the pre-arrival communication and the training of whoever stands at the front. Newcomers experience the sequence as one continuous story, and they remember where the story first went wrong.