How long did the Hubble Ultra Deep Field exposure take?

The Hubble Ultra Deep Field exposure took just under 1 million seconds, or roughly 11 days of accumulated observing time.

The image was assembled from many separate exposures rather than one continuous stare. Hubble observed the same tiny region of sky during two main sessions, from September 23 to October 28, 2003, and from December 4, 2003, to January 15, 2004. The observations used the Advanced Camera for Surveys and were carefully combined into a single deep image.

The program covered about 400 Hubble orbits and produced roughly 800 ACS exposures. Individual exposures typically lasted about 1,200 seconds, while the total time differed by filter. Dithering—slightly shifting the telescope between exposures—helped improve the final resolution and remove detector defects and cosmic-ray hits.

The field appears nearly blank to ordinary telescopes, but its long exposure revealed thousands of galaxies, including some seen as they existed when the universe was only a few hundred million years old. The figure is exposure time, not the calendar span between the observing sessions.