While students may desire to have grades back on submitted assignments immediately, this may not be a reasonable (or doable) turnaround time for you, particularly with assignments like papers or projects (though with Blackboard you can actually create auto-graded assignments—see here or contact Cyberlearning for more info). However, we know from the literature that prompt feedback supports deeper learning as well as the ability to course-correct when performance is insufficient.
It is important that you decide on and explicitly set expectations with students regarding when they can expect to see their assignments returned. You may also wish to explain why the intellectual labor of grading takes more than a few hours, such as the importance of providing thoughtful feedback. Note that while you can set one turnaround time (e.g., within two weeks), you could also consider differentiated return times depending on the type of assignment.
An example syllabus grading statement could say something like:
For a few examples of how faculty have worded response time expectation setting (for both grading and email), see this article from the University of Central Florida.