It works when the DM calls for a check or makes a passive check for you.

The only relevant range is when you request to do an action and the DM calls for you to make a Wisdom(perception) check. Otherwise, there is no clear guideline because environmental or other issues may impact your ability to use it, and those are also up to the DM.

The Lythari

This rare subrace also doesn't seem to be a playable race for PCs. They aren't listed in the playable races and there is no stat block for them. Closest is just being a drow, but that's not quite the same thing as that rare subrace.