As bueller said, yes. But obviously the longer you can leave it the more benefit it will have. That said, I wouldn't change my other plans to accommodate a few extra weeks of BW, plant it as early as possible for your given area(obviously late enough so it doesn't freeze out), and terminate whenever you need to get your other stuff in. If you get 45 days worth of growth it is plenty to see soil benefits.
Another piece of interesting info I happened across: "Buckwheat is nearly three times as effective as barley in extracting phosphorus, and more than 10 times more effective than rye—the poorest P scavenger of the cereal grains."
Here is my take on that fact. If you use a lot of rye and have constantly low P on your soil test, is that low P a product of the fact you are using rye? Wouldn't it make sense to crop behind it to ensure that the P would be more available to subsequent plots? What would it hurt to drill BW into your rye in the spring as it awakens, all you have to do is terminate it when it flowers. Plant diversity is key to getting the most out of your soil, with the fewest inputs. And I'm not saying grow weeds in your clover or not to spray your RR corn, I'm saying be mindful of what you plant behind other crops and you will help your soil in many ways.