Building a custom luxury home would normally take somewhere between 1 year and 2 years. Of course, it depends on the magnitude of the project. But, one year is a bare minimum which any decent custom building firm would take. However, a semi-custom home can be completed quickly, in less than a year. Of course, the building team should be really good. It should have experienced professionals.
To be honest, it is not just the size of the custom home which would determine how long the project would take. The complexity of the project would also play an important role. Sometimes, a large custom home can be constructed quickly if the design is straightforward. At the same time, a smaller custom home can take a longer time to build relatively if the design of the home is complex.
You have to keep in mind that the timeline we are talking about applies only for the construction of the house. It does not take into account the design phase where the architect is involved. So, if you are planning a new custom home project, you will have to take the design phase into consideration.