Standard - For the Standard I think it should be somewhere between 35 and 40 seconds at maximum, I did some tests with placement of towers and 40 seconds is more than enough for another wave.
Competitive - For competitive I think maybe it would be better to make a game mixed difficulty, or a vote to see what difficulty everyone wants, I think the difficulty in this mode should be chosen by vote and not by the host.
Race - For the race mode everyone should go on random but this random is the same for everyone, for everyone has equal chance of winning.
Extreme - I did not understand that the countdown is always 0 in the extreme, and also, life tower should be prohibited in this mode.
would be great if we could come up with another game mode, with heroes perhaps, and where are the powers of the builders?