If you want to look at it in terms of percentages it's pretty easy. You just halve the difference for every generation. So the first 10 generations would be:
style="width: 141px"
|-
| style="height: 20px; width: 77px" | Generation
| style="width: 64px; text-align: center" | %
|-
| style="height: 20px; text-align: center" | 1
| align="right" | 50.00%
|-
| style="height: 20px; text-align: center" | 2
| align="right" | 75.00%
|-
| style="height: 20px; text-align: center" | 3
| align="right" | 87.50%
|-
| style="height: 20px; text-align: center" | 4
| align="right" | 93.75%
|-
| style="height: 20px; text-align: center" | 5
| align="right" | 96.88%
|-
| style="height: 20px; text-align: center" | 6
| align="right" | 98.44%
|-
| style="height: 20px; text-align: center" | 7
| align="right" | 99.22%
|-
| style="height: 20px; text-align: center" | 8
| align="right" | 99.61%
|-
| style="height: 20px; text-align: center" | 9
| align="right" | 99.80%
|-
| style="height: 20px; text-align: center" | 10
| align="right" | 99.90%
|-
After 7th generation you're up over 99% so I'd consider that to be close enough to be called pure again. Plenty of people would disagree with that assessment though.