Hi Kim,
Your question, what is the proper time frame to offer for work turnaround really isn't entirely cut and dry. It's your business, your time and you create the work environment that is best suited for you. This includes setting a work turnaround time you are comfortable with. So if you are concerned/worried that taking on a client who needs something done right then and there isn't going to work for you, then that isn't the right client for you. I have to tell you, that that probably isn't the right client for almost any VA, but that's just my opinion
As your business grows, you are going to find that you have to find a balance between all of the different client work you are doing.. dropping what you are doing for client X because client Y just sent you a fax that has to be entered into a database, well that will just reek havoc on your schedule, and in fact probably throw you off schedule.
What I like to do is schedule out my time each day for each client. I work on Client A for an hour or 2, then on to Client B and so on. I generally provide a turnaround time of a day or two... depending on what the project is of course.
Remember, it is your business... you get to decide what is most comfortable for you in terms of time and you get to decide who you want to work with too.