My program was not entirely online, but I was able to arrange my clinical rotations in my home town, so perhaps I can be helpful.
The most basic way to get your hours in is to look at how many hours you need to work to fulfill the obligations for the course, how many weeks you have to do them during the semester, and then work with your preceptor's schedule for office hours.
In other words, if you have to do 300 clinical hours in an 18 week semester, you can figure that a week or so of that will be "wasted" because you may not be able to start of Day One of the semester OR you may need to build in a cushion in case you get sick, the preceptor gets sick, etc. So divide 300 by 17 to give yourself some wiggle room. That would indicate you need to be in clinic for 17-18 hours per week. Take that number and look at your preceptor's schedule. What days/hours is he/she normally in clinic? Is he/she planning any time off during the semester for vacation or CME? And so forth.
In addition I assume you will be doing some research, writing scholarly papers, etc. during the semester. You will have to determine at that point whether or not you can work or how much. I did not work during my clinical rotations...I had friends who tried to work; one was a long-time employee of the VA and so was able to work only 3 or 4 days/month since she had banked LOADS of time-off.