What is the amount of test data needed to evaluate a CNN?

I have an image dataset of about 400 images. 70% of these data points were used for training, 15% for validation, and 15% for testing. I am using the 70% to train a CNN-based binary classifier. I augmented the training data to around 8000 images. That makes my test set really small in comparison. Is that ok, and what is considered a decent size of images for a test set?