One mole of a substance is approximately 6.02 * 10^23 molecules. Imagine each grain is a molecule - you are counting 100 per 88 seconds, hence 1 per 0.88s. To find the time, multiply (6.02 * 10^23) by 0.88, gaining a time of (5.2976 * 10^23)s. Divide by 3600 to gain the number of hours, then by 24 to get the days, then finally by 365.25 to get the years - the answer is (1.7 * 10^16) years, or 17000000000000000 years