How long to brute force

Suppose that our key value is only as big as 10,000,000,000,000,000,000,000,000. We know that there are 176,846,309,399,143,769,411,680 prime numbers less than that value. If we have to try them all (true brute force), how long would it take us if we had a computer that could try 1 million keys every second? Report your answer in years.

I know the answer is 93.4 or 46.7 billion (since we only need to run half of it); however, I am not sure the process.