The two standard heights for corner guards are 4ft and 8ft (or whatever the full height of your ceiling is). And which one you should use is mostly aesthetics.
The Part That's Not Aesthetics
Most things that cause damage to your corners are below 4ft (e.g. carts, luggage, wheelchairs). If there is anything above 4ft that will cause damage to your corners then you would of course want to use an 8ft corner guard.
The Part That is Aesthetics
There are three aesthetic considerations when deciding between 4th and 8ft.
First, if you already have corner guards, what height are they? You want to keep a similar aesthetic throughout your building and having a consistent height helps to provide a cohesive interior design.
Second, often there will be wall sheet or wall coverings that goes from the floor to 4ft. And to match that aesthetic change with a corner guard is an elegant solution.
Third, do you like it? Some owners simply like the look of a continuous color from floor to ceiling. Also every interior color palette will have multiple colors and selecting a complementary color to the wall color from the palette is an beautiful option.