Building a custom home is quite time consuming. Sometimes, it can take years to complete the construction of a custom home. At the very least, it can take 2 years to build a luxury custom home of considerable size. If a custom home based on a model home, then it would probably not take more time. If you are building a home without any reference at all, then it can be really time consuming and it depends upon how well and how quickly the design of the home is done.
Planning is key to completing a custom home as fast as possible. Detailed planning is required, not just surface level planning.
Even before the start of the project, it is worth it to decide the designer and the builder you are going to be using for the project and have a detailed discussion with them and plan every detail of the project beforehand including the budget and the timelines. Doing this can make your life a lot easier and the builder would be clear on the expectations provided you hire a competitive custom home builder.