Rather than use an oversimplified timer that requires you to re-enter the time each change of speech, use the Speech and Debate Timer app which makes keeping track of speeches and debates easy.
Features:
-Automatic times (no need to set the speech durations unless you want to)
-Visual time circle
-Large, easy-to-read and simple design
-Optional audible and visual alert
-Modes for different speeches and debates
-Ability to set custom times