According to the blog Householdname.com the answer is six months!
Photographer Justin Quinnell strapped a pinhole camera made from an empty drinks can to a telephone pole between December 19, 2007 and June 21, 2008 (A 15,552,000 second exposure!) to capture this image of the Clifton Suspension Bridge in Bristol, UK.
‘Solargraph’ shows six months of the sun’s luminescent trails and its subtle change of course caused by the earth’s movement in orbit. The lowest arc being the first day of exposure on the Winter solstice, while the top curves were captured mid-Summer.
(Dotted lines of light are the result of overcast days when the sun struggled to penetrate the cloud.)
The original post is here.
More pinhole photographs by Justin can be found here.
EXTRA CREDIT: Here’s how to make your own!