Since you didn't give an amount of time, we can't give an exact value. However, I can help you with the steps so you can get the correct answer.
First, you need to find the correct z-score. Then, use a standard normal distribution table to find the percent.
Let the time that you are looking for equal X.
The formula for your z-score is:
(x - 8.7) / 2.1 = z-score