Always wondered why this hasn't been implemented yet. Knowing how long the game lasted is really useful when you're looking back at a past game. In particular, amount of gold earned and number of minions slain are a LOT more meaningful when you know how long the game was. It really give you a much better picture of how you did in that game, how well you cs'ed, how much you contributed to fights in the amount of time the game lasted.
More suggestions: this can also be implemented in ranked, where you can see how well you cs for each particular champion. More importantly, this give you an idea of how well you are using your champion if it is an early game/ late game champion. For example, average game time in a winning game as Shaco should be a lot shorter than one where you played as Ashe.
It would be even better if game length can be shown separately for won and lost games. This can really help analyze play and improve.
just my 2 cents.
Edit: And also, knowing the game length also tells you about your items, like how well they worked in a long/short game, and were you ahead or behind on farm, etc.
