When I was grooming, we used to lightly tip some dogs nails everyweek with no problem.
It depends with some dogs, some may wear them down quicker then others, like if they run on concrete or dig alot.
You may just need to experiment, but before you do.. make sure you have quick stop on hand. If you do accidentally cut her, dont blame yourself, it happens. The more stressed out you get over accidentally making her bleed, the worse it is for the doggie.
Just do little pieces at a time and look at the center of the nail (I know its hard to do with doggies that dont like it). I know with my dog with black nails, I can look under her nail and see where her nail goes from nail to quick (the quick looks thicker and more brownish).
Its hard to explain but since I have alot of experience at it, I can tell when I am close to the quick by looking at the nail itself after I tipped it, the color changes.
Heres a good link that is describing that color change:
http://www.vetmed.wsu.edu/ClientED/dog_nails.asp
I do Turbies every 2 weeks.