its pretty clearly stated at the game lobby screen as soon as you enter on the right side.. i'd say just pay more attention..
a spin-off of this suggestion could be that during the 45,60,90 timed games you have a warning beep at the 30 second mark..
I'd say maybe moving the warning beep to the 15 second mark on all game types.. because then its more of a warning system and will activate accordingly during the 30 second games as well..
(or maybe even a second warning beep at 15 seconds for double the warning)