I experimented with this when I first got here. Once, I was surprised it even let me set the rate... SBD was less valuable compared to steem, then and I put a ratio of 1:1...surprisingly, I did get the steem I placed an order for. It happened one other time, but after setting my rate, the transaction refused to go through. Haha
It's very handy and sweet.
It should always let you set the rate. Whether anyone fills the order is another question. But you can let your money sit in there for as long as you like in hopes that someone does.