Ok so I don't see anything about the amount of food the dog eat everyday, so I will take the average medium dog (20-60 pounds)
A 20-60 pounds dog will need around 4-12 ounces of dog food (0.25-0.75lb) everyday.
So, if she divide everyday equally, the dog should receive 20 : 14 = approx. 1.4
So, a 20lb bag would be enough for 2 weeks.