Its one of the important things, its not the be all and end all you seem to think.
Case in point - Mass Effect. It goes on, and on, and on... and its boring.
Ikaruga - only has 5 levels, you can play through it, if you are good enough, in under an hour, and its a classic.
Total play time is an issue, speshly for a game of this type, but there are many other things to take into consideration.