Completed using Gamemaker and Paint.net in a total of 16 hours due to work and laziness.. No sound, simple graphics, simple gameplay. 5 minutes of guarenteed 'fun'. Source included in the .rar as .gmk file.
I only took time to play as a plant. The standard gameplay got repetative after a while, but the boss suprised me the first time. Also, 16 hours is indeed not the ideal time you could have put into this game. As soon as you have a bug-free game and an engine it should take about 15 minutes to double your minutes of guaranteed "fun".
There's a serious bug; I played as a bacteria and after my health reached negative numbers I could just keep on playing. Also, the screen that explains the controls only appears once you beat the first wave, which isn't too useful. The gameplay is pretty repetitive, not gonna lie.
Other than that, I can't rate your game?