Mine comes on at around 64-65 litres used (15-16 remaining).
If I'm driving constantly at the same rate, like on a long trip, I can do the math to find out how far reserve will last - just plug the figures into this:
km traveled so far times 15 divided by 65 = km to go
Example: fuel warning light comes on at 600km: 600 * 15 / 65 = 138km to go
I wouldn't take it all the way of course, but you'd know you could do about 100km more.