There are many bigger problems with The Order than length.
Look at the new Wolfenstein. That's the marketing and release style The Order needed. But I think Sony badly needed to recoup something for the five years they spent building this engine and game.
I think Wolfenstein is a solid example of what The Order could have been.
Depending on play style, it is anywhere from 2-4 times the length of The Order, and has a tangibly different alternate storyline, meaning you've genuine incentive to replay the whole thing.
I have not yet played The Order as rental is not easily available in Australia, but by most accounts Wolfenstein's story is a lot better too. That said, it isn't as pretty on consoles (but of course, a high end PC negates that).
I also cannot for the life of me understand all these people that specifically talk about The Order, and then go on to utter a variation of "I'd rather play a short and fun game than a long boring game." I take issue with this for two reasons. The first is that by many accounts, The Order is a short, boring game, not a short, fun game. The second reason is more important. The rest of us are not playing long, boring games. We're playing long, fun games.