How long does Apple permit a background task to run?

I have to upload an array of image files to database, therefore, I stumbled upon Apple's background execution guide to make sure the app still uploads the data when user suspends or terminates my app.

But in the desciption, it says giving it a little extra time to finish its work if we call beginBackgroundTaskWithName:expirationHandler: or beginBackgroundTaskWithExpirationHandler: to start a background task.

How long is little extra time precisely?