I used to time my poses with my Palm, using a nifty little add-on called PocketDoan, which is a highly configurable timer. It has some perfect sounds on it, like chimes, bells, and wooden clackers, which are much nicer alerts than buzzers and beeps. I still use it for Savasana.
But now in most poses I use counted breaths. Ten breaths (that’s one inhalation and one exhalation) are usually 45-60 seconds for me. It’s a calming way to hold the pose and keep my attention in the right place.