The time factor will depend on the following considerations.
How long it takes the client and myself to decide on a final basic plan and design. Although this sounds simple, some of my clients are frequently out of town on business, or even out of the country. Factors such as these will add time to the process.
How long the engineer takes to sign off on the plans.
Should any building lines or servitudes be encroached, the signatures of the neighbours will be needed, and they might not be available to give such signatures. The plans might also be sent to the legal department of the council, which will also add extra time to the process.
The town council: As sometimes happens, the council may be inundated with building plans, and the process may be delayed by days or even weeks.
Which departments need to sign off on a building plan. Some departments, such as the fire department, only sees the public on certain days, and then only between certain times.
Any other factors, sometimes foreseen and sometimes unforeseen.