Highly depends.
If the quality's excellent like Grisaia, I don't mind how long it is. Great character interaction/dynamics is a must.
If it's poor/ok quality like Tsukihime (which nicely keeps the common route a minimum), I'd prefer it short and to the point.
(Edit) Didn't mean to say that Tsukihime common was awful lol. I thought that's what the ok was for? Readable quality. I meant that I don't want a common route like this to be medium or longer, which it didn't do ;D