It could take a minimum of 12 months to complete a custom home. More often than not, it could well exceed 12 months. It actually depends on the complexity of the floor plan and design of the custom home. An accurate timeline can be given by your builder. There are various factors which determine the timeline.
The project time usually includes right from the plot hunting phase. But again, depends on your requirements. If you already have the plot ready, then you can exclude that phase. The design phase could also be included in the timeline if it is also handled by the builder. But, in most cases, the time when actual construction begins is usually considered as the beginning of the project and the clock starts ticking.
The working relationship which you have with the builder also determines the time it takes for completion. You need to be in constant touch with the builder and get updates on the progress. You should never leave anything till the last moment.
The weather conditions also determine how long the project takes to complete. For example, if there is constant rain, the project can get delayed.