Your right it could be as late as dec 2018. But how would the company be able to guess that 10% of profits would allow them 100,000,000 reduction if it was so far away and so much could be different By then such as the pps?

I say it's much much sooner because for the company to calculate it accurately for both pps price and profit to say they can buy back 100 mil shares using 10% profit means its sooner rather than later.