If you are considering buying a building or doing a build-to-suit, you can add an extra few months to 18 months to your schedule. Buying a building can take extra time because you may have to get a zoning change and a change in use may trigger compliance with "grandfathered" codes. For example, if you convert a day care building to an office use, that may trigger the requirement to install a sprinkler system. If the building had been used for another day care, no sprinkler may be required.
Once you start talking about ground-up construction as in a build-to-suite (BTS), you can easily add 18 months or more to the schedule. That's because you may have to create and file a plat, change the zoning or get a variance, create and submit plans for city approval, and then construction, of course. And this construction isn't like interior construction. Now you have rain, snow, cold and heat to contend with and they cause delays.
None of this is intended to scare you off from your intended project. It's just to reinforce my first comment - START EARLY.