Whole projects, from start to finish can be really long. Some big space missions, like Mars rovers, can last 10+ years. Other projects are much smaller, maybe a couple of years on average.
But generally I get to work on lots of projects at the same time, so every few months I’ll be doing something completely different. It’s great as I think I’d get a bit bored if I only worked on one project for 10 years!
Most projects we do at work are 1-3 years. I started one project last October that has to be done this coming October (if i can get it all done! it’s harder than you think to finish projects on time…) and another project i began in 2010 and it’s finishing later this year too.
Gosh – there’s no one answer to that! Our project to build the accelerator in the first place lasted five or six years. Individual projects to improve some part of the machine might run from a year, to just a few weeks.