Colin Creevey (a Muggle-born) has a brother who is also a Muggle-born. How common is this and how quickly would the magical ancestry show in a Muggle family?
Taken off Harry Potter wikia in reference to J.K. Rowling and the Live Chat, Bloomsbury.com, July 30, 2007 (2.00-3.00pm BST):
Muggle-born witches and wizards are descended from Squibs who married Muggles: the magical ability unexpectedly resurfaces after many generations.
I found a list of known Muggle-borns from Hogwarts and the Creevey brothers were the only siblings. I think it is highly unlikely that having more than one wizard in a Muggle family is common. Think of Lily and Petunia, or Dean Thomas and his sisters. Hermione was even supposed to have a Muggle little sister, but J.K. Rowling cut her out. So I am concluding that although descendants of Squibs will eventually have magic, it rarely happens in the same generation.