LEDs have become more efficient over the past 25 years.
Originally a red LED emitted 17mcd @20mA. These LEDs now emit 1,000mcd to 20,000mcd @20mA. This means you can lower the current and still produce illumination.
Some LEDs operate on a current as low as 1mA. Some high-bright white LEDs are TOO BRIGHT to look at and will hurt your eyes. It is impossible to give any information on the output required for any particular application.
Old LEDs require 15mA to produce a dull illumination that does not emit out of the opaque red/green/orange LED and is just a waste of 15mA.
You can get a high-bright LED to produce a higher brightness at 1mA to 5mA and save a lot of battery energy. Design all your projects using high-bright LEDs with a current of 1mA to 10mA.