Both human listeners and machines need to adapt their sound categories whenever a new speaker is encountered. This perceptual learning is driven by lexical information. In previous work, we have shown that deep neural network-based (DNN) ASR systems can learn to adapt their phoneme category boundaries from a few labeled examples after exposure (i.e., training) to ambiguous sounds, as humans have been found to do. Here, we investigate the time-course of phoneme category adaptation in a DNN in more detail, with the ultimate aim to investigate the DNN’s ability to serve as a model of human perceptual learning. We do so by providing the DNN with an increasing number of ambiguous retraining tokens (in 10 bins of 4 ambiguous items), and comparing classification accuracy on the ambiguous items in a held-out test set for the different bins. Results showed that DNNs, similar to human listeners, show a step-like function: The DNNs show perceptual learning already after the first bin (only 4 tokens of the ambiguous phone), with little further adaptation for subsequent bins. In follow-up research, we plan to test specific predictions made by the DNN about human speech processing.