There a number of countdown timers out there - see for example http://www.m62.net/powerpoint-slides...erpoint-timer/.

However why not just make your own.

Create two WordArt objects, one (Tens) containing 543210, the other (Ones) 9876543210 and adjust the character spacing so all the characters overlap.
For the Tens object add a custom animation for entrance which starts immediately and where you Animate text "By Letter" with 10 seconds delay between letters. For the same Tens object add a custom animation for exit which starts after 10 seconds and where you Animate text "By Letter" with 10 seconds delay between letters.
For the Ones object add a custom animation for entrance which starts immediately and where you Animate text "By Letter" with 1 seconds delay between letters. For the same Ones object add a custom animation for exit which starts after 1 seconds and where you Animate text "By Letter" with 1 seconds delay between letters.
For the Ones object add a second custom animation for entrance which starts after 10 and where you Animate text "By Letter" with 1 seconds delay between letters. For the same Ones object add a second custom animation for exit which starts after 11 seconds and where you Animate text "By Letter" with 1 seconds delay between letters.
For the Ones object add additional entrance animations at 20, 30, 40 and 50 seconds and additional exit animations at 21, 31, 41 and 51 seconds.
Ensure all the custom animations start "with Previous"

This gives you a 60 second timer. If you want a longer timer just duplicate this slide, add word art giving the minutes and move between slides after a minute. If you need less than a minute just delete the characters from the Tens object i.e. for 30 seconds delete 5, 4 and 3.