Sorry for the delay, but here’s how to do a countdown fairly quickly using “find and replace” and “copy and paste” in PowerPoint. This works up to 99 minutes, but you’ll have to delete some (or start on a certain slide) if you want to start short of the even 10 minute marks. This is the thirty minute version:

1) Create a slide with “WX:YZ” in place of the numbers.
2) Copy and paste until you get 10 slides.
3) Replace the “Z” in the first slide (manually) with 9.
4) Repeat until you get to 0.
5) Select all; copy.
6) Find and replace “Y” with 5.
7) Paste.
8) Repeat steps 6 & 7 until you get to 0.
9) Select all; copy.
10) Find and replace “X” with 9.
11) Paste.
12) Repeat steps 10 & 11 until you get to 0.
13) Copy.
14) Find and replace “W” with 2.
15) Paste.
16) Repeat steps 13 & 14 until you get to 0.
17) Switch to the slide sorter view.
18) Set the slides to automatically switch after 1 second each.

You’re done. Delete what you don’t need to get to 25 (or whatever) minutes.