Probably longer than you think.

For a typical large extension and reconfiguration project it may be 8-12 months before you even get started on site, allowing for proper briefing and design development, waiting for planning (a minimum of 8 weeks needs to be allowed for this), and then working out all of the detail and coordinating the work of other consultants like engineers and other specialists. Then you need to find the right builder at the right cost before you can start.

The work on site might take 4-6 months, or even longer if the conditions are complicated or you are doing something interesting.