We ran a sample validation test on Mashable.com with a Ford advertisement. Below are the results split up by the number of respondents. You can see that the heat maps with low base sizes contains large differences, but at about 20 respondents and up the heat map remains mainly the same.
The chart below shows that the numbers stay at a constant rate from 25 respondents and up.
With that being said, at 30 your results will be representative of your sample.