Play a song on your iPod. Wear your headphones and be sure to put the iPod, itself in a pocket on the inside of your clothes.
Many iPods (and other players) have little remote controls right on the headphone cord that lets you select a song or playlist.
"Over the Hills and Far Away" by Led Zeppelin is 4:50 long.
"Misty Mountain Top" is 4:40.
Just string together a bunch of songs in a playlist, one for each step of your process.
If you want to develop for a shorter time, pick a shorter song like "Going to California" which is only 3:30 long. If you wanted to develop for a long time, maybe "Stairway to Heaven."
Pick your favorite songs and, not only will it help you pass the time in the darkroom, the rhythm will help you keep your agitation regimen consistent, too.