That's an excellent question, and the answer depends how you define a "project."

>I work almost entirely on one big project, but there might be a number of experiments I use to study different aspects of that project.

For example, my project is to investigate and describe a particular protein from the bacterium Bordetella (the one that causes whooping cough in people and kennel cough in dogs). Within that project, though, I'm trying to find out when and under what conditions the protein is made by the cells, where the protein goes after it's made, what happens to the bacteria when you remove the gene for this protein (so the protein can no longer be made at all), and whether animals infected with Bordetella without my protein get sicker or less sick.(Whew! That sounds like a lot!) So I guess that's all one project.

Researchers in my position might keep a little side project or two going as well. Since it's just a side project, they might pick one that's more risky or less likely to work out well, or they might have a side project to keep them busy during lulls in their main project.

Technicians and assistants might have only one or two of those "sub-projects." If you become the head of a laboratory, then you might have five or six (or even more) of these big projects going on in your lab. You wouldn't do all the work yourself, but you'd be in charge of all of them.

A project like my Bordetella protein might last anywhere from two to six years, depending on how far I take each different sub-project. Any one of those sub-projects could be pursued for a year or more--it largely depends on how the experiments go and what results you get. If you make a career out of research, you could pursue a project like this, following new questions as they arise, for the rest of your career! I hope that's helpful, and good luck.