I'm a musician as well as a photographer. We musical types have a trained sensibility for tempo speeds. March tempo (your favorite Sousa march... I like the Washington Post) is 120 beats per minute. That equals 2 beats per second or one second per 2-beat measure.

Counting measures is also second nature to trained musicians, especially brass and woodwind players who sit around in the orchestra while the strings are doing all the playing.

So... counting measures at march tempo equals counting seconds. I tested myself not long ago with 300 measures of "mental marching" and came up withing a couple of seconds of exactly 5 minutes.

For longer camera exposures and print exposures of 20+ seconds, I feel I am pretty accurate.

Note, however, that I only do this when 1) I'm in the field not wearing a watch with a sweep second hand or 2) am making some down and dirty test prints where being 5-10% off on time won't make a lot of difference.

On the other hand, I think with practice I could be repeatably accurate within the tolerances needed for printing.

If only there were some catchy words for the "Stars and Stripes Forever"... Oh wait.. "Be kind to your friends in the swamp/for a duck may be somebody's mother...."

Have fun