I only ever do it for raids/primals and even at this point, it's something like 6 or 7 seconds. If I ran savage or ultimates, i'd probably up this to 10-15seconds.
I wouldn't use it in dungeons as there is no need

I've seen both sides of the coin when it comes to countdown timer complaints. Some say "what is the point in this" another has said "Can you do a timer instead of running in like a scrub"

There is no harm in using it, people will only get upset if it's too long and they feel time is being wasted.