It is absolutely crucial in online courses that turn-around for feedback/grading be quick and happen with a defined time frame. That could be 12 hours, could be 24 hours, could be 48 hours, but it shouldn't be longer.
Gone are the days when students would hand a paper in class one week and get back a graded paper two weeks later. Because students are often working around their own schedules, and at their own pace online, you've got to get feedback back to them swiftly so that they can stay on their schedules.
The feedback doesn't have to be exhaustive; you can also offer more to students if they request it, but it does have to be speedy. That also let's students know that you are engaged with them in the learning process.