How long can a loading screen be before it starts creating a negative user experience?

1. How long should a loading screen be?
2. What happens behind a loading screen?

How long should a loading screen be?

The simplest form of animated indicators is a loading spinner. This type of visual feedback just states the fact that user has to wait, but doesn't say how long they should wait. As a general rule, you should use this type of progress indicator for fast actions (2–10 seconds).

What happens behind a loading screen?

When you see a loading screen, what's usually going on is that it's hiding the steps being taken by the application: it's what you see when data is being transferred and when the program is being initialized.