We Judge a Website in 50 Milliseconds
Research from Carleton University indicates that we make flash judgments of websites in about a twentieth of a second:
Lindgaard and her team presented volunteers with the briefest glimpses of web pages previously rated as being either easy on the eye or particularly jarring, and asked them to rate the websites on a sliding scale of visual appeal. Even though the images flashed up for just 50 milliseconds, roughly the duration of a single frame of standard television footage, their verdicts tallied well with judgements made after a longer period of scrutiny.
No pressure there, web designers.