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 is a loading state?
Loading states are just that — small pockets of time that should be judiciously used to provide the user with visibility on what the system is doing. This is key in helping users feel in control, making them aware of the context at all times, and reassuring them that the right task is effectively being worked on.