It's something I've done for over 25 years. Some projects have spanned that period others have been 2 - 5 years before the works exhibited.
I spent 2 years photographing an abandoned canal, followed by 5 years shooting Industrial Landscapes in an Urban area, and my main projects at the moment have been shot over another 5 year period. I tend to set longerterm aims/goals with Exhibtions in mind, these often need to be arraged with Galleries 2-3 years in advance.
All my projects actually form a more complex body of inter-related work, and I've gone back to University twice to help contextualise the work.