There are few studies of model-based survival projections using early empirical results for estimating long-term survival. Utilizing Early Breast Cancer Trialists' Collaborative Group (EBCTCG) data, a Markov model was generated to compare empirical results with those modeled beyond the empirical result time horizon in estrogen receptor (ER)-positive early-stage breast cancer (ESBC). Modeling 15-year survival based on 5- and 10-year EBCTCG data resulted in an average error estimate in breast cancer mortality of 0.75% [range -0.83 to 2.19%]. Although modeling life expectancy differences ranged from an underestimate of -7.93% to an overestimate of 12.64%, over the span of 15 years this corresponded to a loss of 18 days or a gain of 40 days of life. Reliable early survival data may be used to generate models that accurately estimate 15-year survival in ER-positive ESBC. Whether early survival data can be employed over the lifetime horizon remains to be demonstrated.