Trying to understand URLSession data caching

Question

I am downloading images from a website using the code below which caches images. Do I need to save images in the application directory so that they can be saved forever?

After running this code, I can turn on my device in airplane mode, restart it and open the application, and the images are still there. What bothers me. I thought caching was temporary? Where is this image stored and, most importantly, how long?

I read the Apple URL Programming Guide and it did not answer these questions.

---

func getImageFromWeb(_ urlString: String, closure: @escaping (UIImage?) -> Void) {
    guard let url = URL(string: urlString) else {
        return closure(nil)
    }

    let task = URLSession(configuration: .default).dataTask(with: url) { (data, response, error) in
        guard error == nil else {
            print("error: \(String(describing: error))")
            return closure(nil)
        }
        guard response != nil else {
            print("no response")
            return closure(nil)
        }
        guard data != nil else {
            print("no data")
            return closure(nil)
        }
        DispatchQueue.main.async {
            closure(UIImage(data: data!))
        }
    }; task.resume()
}

Function call:

getImageFromWeb("http://www.apple.com/euro/ios/ios8/a/generic/images/og.png") { (image) in
    if let image = image {
        // Do something with image
    }
}