Back when I worked in radio we timed spots with a sweep second hand on a clock on the wall (or near the board). Spots were generally 29.5 seconds then. It depended on how you watched the clock as your read the copy. And running the board, you listened for the outcue and manually started the next spot. That gave you a little break between spots.
Now with digital recording you can watch a timeline and hot 29.95 seconds if you want to. And the computer plays the next file/spot as soon as the first file/spot has finished. Computers tightened things up quite a bit.