Over the years, I have found that structuring an online course  in regards to time from day one is best. Most online students balance work, family, and possibly other courses.  Share with students your avaibility at times that best fit you and the student. For example, I'm a morning person so I am most productive between 7 am - noon.  I shared with students that I will review assignments, and reply to any emails  between those times on specified days, eg Mondays and Wednesdays. I also request to allow 48 hours for feedback. As instructors, we also have other responsbilities to fulfill.  Students can email at any time and I will  reply with the time frame desired. 

I am also flexible with due dates.  Unexpected things happen to us all, so I try to be compassionate;however,  consequences are applied as well, such as points deducted for being late.  I request for documentation (doctor's note, court, military leave, etc)  as well to justify any extensions.