Film? Slide? Negative? Digital?
Film/Slide: you can expose up to 30 minutes (my personal experience) if you use a Fuji Provia 100F without increasing the grain.
Film/Negative: not enough experience, just starting with Fuji Pro 400 H.
Digital: depends on the sensor. The sensor of i.e. the Nikon D600 appears to handle 5 minutes exposure @ 100 ISO without producing grain (again my personal experience). Longer exposure times not yet tested. But you have to turn off all in-camera options for sharpening, D-Lighting, noise reduction, color effects, etc.