Lactation models were used to fit and forecast extended lactations to determine if cows suitable for extended lactations could be identified within their voluntary waiting period (VWP). Models were fitted to a dataset of daily milk yields (MY) (n = 651) from extended lactations > 305 days in milk (DIM). Models predicted 305-d MY after fits to data from only the first i DIM, where i was incremented from 30 to 300 DIM in weekly intervals. At least 90 DIM of data was needed by the model for a satisfactory fit. However, using 120 DIM of data was recommended for more accurate forecasting. Beyond 120-d there was a lessening impact of additional data on the improvement of fit statistics. Since a 120-d VWP is not optimal for all animals, it was suggested that model forecasts should be used as a tool to optimize lactation length only in suitable animals.