Finding out when cracks become unstable is at the heart of fracture mechanics. Cracks often grow by avalanches and when a sample fails depends on its past avalanche history. We study the prediction of sample failure in creep fracture under a constant applied stress and induced by initial flaws. Individual samples exhibit fluctuations around a typical rheological response or creep curve. Predictions using the acoustic emission from the intermittent crack growth are not feasible until well beyond the sample-dependent minimum strain rate. Using an optical speckle analysis technique, we show that predictability is possible later because of the growth of the fracture process zone.