I agree with Lizzy's rule-of-thumb, too.
What's the point of clinical experience? </rhetorical question> Supposedly, you are using clinical experience to gain personal exposure to the care of patients and the practice of medicine. You should know if you like/can handle sick, gross, grumpy people. You should also learn something about the role of the physician, since that is what you're wanting to be, as well as having an appreciation for how physician orders actually get accomplished.
In your application you're probably going to say things about how you know medicine is for you, or that you were called to medicine, etc, ad nauseam. Clinical experience gives these statements some credibility. You didn't see it on TV. You got as close as you could as an undergraduate and you loved it and you want more.
So, "office assisting" in a medical office is valuable experience, but I wouldn't let that be my only experience if possible. It would be better if you could put yourself out there a little more and get closer to the patients. It would awesome if you could have some limited responsibility in their care, too (along the lines of scribing in the ER, tech'ing on the floors, drawing blood for the lab, etc.).
I personally, care less about the number of hours and more about the experience you had and what you got from it and how you are able to communicate that to me.
