Well, that depends.  If you get lucky, it could take less than an hour.  However, if you plan on doing a thorough search of all available possbilities.  Then narrow them down so you are 100% sure you've chosen the best possible brand-name-matching .com domain name, than this step alone will take several hours.  The entire process should be spread over about 3 days with a few hours every day.  That way you've done all of your due diligence and are sure of your decision