This all depends on the requirements gathering and the final scope of work. If frontend design is simple, requirements are default Magento and no heavy customizations are needed, then it will typically take 10-14 weeks, depending on a few factors. If the site needs a UX and creative design and there is a heavy integration, then the schedule can take 20-30 weeks. A large portion of a schedule can be controlled by your business. If there is clear understanding on needs vs. wants and expectations are set, then a real-world schedule can be forecasted during early conversations with sales.