Hi Brett, I would think it really depends on the actual project you are considering.
Take for example the "Greeter" project, you could volunteer a couple of hours a week depending on your personal schedule. You would know your schedule and most times stick to it and sign out and be on your way.
Now the "Data Doctor" project that works with database errors report may take more time then you started out in a day to work on because of the complexity of the project. You might think, "well I will make a few corrections, maybe an hour or so", and find out you are there a half a day, that happens to me alot.
I have a calendar that I work off of each week. I have my hours posted on it for required duties like the greeter and mentor program and then as my outside commitments arise, I look at the calendar and see if something needs changing or ask someone to fill in for me etc. Of course I am retired so I don't have a set work schedule that I would need to take in consideration when volunteering.
Many projects you would work at your own pace doing a specific task, rather then work a particular schedule like the greeter project. A project leader would be able to tell you in advance what is expected and usually there is a list of tasks on the project page itself that would give you an idea how much time would be needed in volunteering.
Hope that makes sense. But whatever you decide, all volunteer effects are so appreciated and gives you opportunities to work with others.