Bearfruit

It depends on what you mean by “implement.” If you mean have a site running with a standard theme? Not long, probably 15m. To create a theme? Drupal is easier than most since you only need a single php file and there are many existing options to use as models, so just a little longer than is typical of your designers. However, a big site will take much planning and ideally a test site where you can do some trial and error. For a site like Ubuntu.com you’d want to allow at least 2 months.