Pah, I'd rather play a three hour long game with endless replay value than a 50 hour RPG which I'd never touch again after completion...
It took me little over a few hours to complete Super Mario World SNES for the first time. Then I realised there were numerous secret exits and cool level designs.
That was 15 years ago. I still hammer it frequently today, have two copies on SNES and a Wii VC version, have played it through ridiculously too many times, about 2000+ hours I'd estimate! I just love that game so much.
Point is, if the gameplay is good and level design fun enough, ultimately I don't care how long it takes to finish it one time through: for me, it's all about how much i enjoy playing it again and again over many years, hopefully this will be the case with multiplayer extending this yet further.
Tell me though, ignorin lifespan and lenght, how good is the game? Purely on basis of how enjoyable the levels first time through, and how many 'wow' or 'incredible' momnets?