A game should take at least 40-45 minutes. 50-60 is rather perfect, that means that 1,2,3 have 1-4 good items and 2 semi good items, supports 1 good item, that allows some nice fights. With everyone 6 slots the game just has 1 final fight, then its usually over because creeps and towers are usually too fucking weak and go down too quickly.
If they only need some seconds to take down a t4 tower theres something really wrong. Towers are too weak in general. In dota1 there was an easy mode and a normal mode. dota2 feels like easy mode anyway, weak towers and high gold income. Some patches are really needed.
And I surely prefer a 60min game with 20mins extra farming over a 20min game where people rush and 1 side absolutely gets crushed and has no chance at all, because they either suck or fail by heavy mistakes (or picks).