This is of course entirely depending upon the size and complexity of your site. It is also depending upon the resources of your web design company, i.e. a "one man band" will, at least in theory, take three times as much time to complete the job as a company that can put three designers on one job.

Even a small site of say a dozen pages would probably take up to a month from the very beginning, including the first meetings and discussions and agreeing upon a concept and a look, until it is completed. Remember that the site should be tested and verified and then approved by you before being launched.