Projects are important for any photographer. They can give you direction, motivation and help the photographer see new possibilities and give them something to aim for. Certain photography projects can take the photographer out of their comfort zone opening their eyes to new styles of photography and learning new techniques.
There is no set rule as to how long a project must be. They can be a few hours or days to years or even decades. I feel short projects are particularly useful when the photographer finds him or herself in a creative rut. It can help them rethink their approach and essentially get their photographic mojo back.