Since we know that things usually don't change instantly (for example, a cat won't suddenly change into a dog), if we assume 10fps vision, 1500 pictures of cats would mean looking at a cat for 2 and a half minutes total in 5 years. And since we know cats won't change into something else, if we see a cat walking somewhere, we'll still know it's a cat, giving us the labels we need for the training.

I think that if we assume 30fps (which still seems kind of low), and we assume that the human looks at a cat for 15 minutes (which still isn't much) that's already 27000 pictures.