Technically, your farm exists as soon as you create the configuration database with the first Complete installation type. Depending on your design, you can immediately begin to expand your farm with additional servers to support WFE or other functions. You might need to move search, index, personal sites host, Excel services, and other services to separate hardware to reduce workload on the WFE.
Alternatively, your farm can continue for some time as a single server hosting all services (other than SQL Server) until performance monitoring indicates a need to expand. We recommend that the SQL server always be on a separate server except in very small production implementations or development and staging implementations.