I started using a timer about ten years ago for mundane tasks (getting done a chunk of unpleasant work, or for cooking) and these days I'll get up from my desk to go check on the oven right before the timer rings.
It works regardless of the length of the timer, and even when I don't know exactly how many minutes it is (twisty analog oven dial).
I don't consciously know how much time is left, but somehow I do know when "it's time."
Haven't tested this on longer timescales (hours) though, but I did meet a guy who said he always knew what time it was because he used to work for a TV station doing the countdown til they go on air.