Headers can take anywhere from 6 to 12 hours depending on the method used, the brand/type of header, and the experience level of the installers. If they drop the K-member the job can be done by the average gear head in about 8 hours.
U.M.
P.S. You do know that headers are not the best "Bang-for-the-buck" modification for these cars right? Plenty of 500-600RWHP Cobra's running around with the STOCK exhaust logs.