That really depends on the project! Some can only take a few weeks before they wrap up but others can last a lot longer if they are very interesting. It also depends on if they are independent projects or a group project – either way it is never dull!
My overall PhD project will take me about 2 years to complete. But in that project I have divided it up into smaller chunks to work on which will take around 3/4 months at a time making it much more manageable :).