For much of 2017, shares of Apple (NASDAQ: AAPL) surged higher, driven primarily by the growth of the company's services business and excitement about the new iPhone X. However, various negative reports about iPhone X demand have surfaced in the past month, taking some of the shine off the stock.

One analyst thinks that a rising tide of share buybacks -- enabled by the recently passed tax reform bill -- could propel Apple stock higher in 2018. In fact, Steven Milunovich of UBS expects Apple to buy back $122 billion of stock over the next two years. However, that eye-popping figure isn't as impressive as it might seem at first glance.

There will be a lot of cash available

As of the end of September, Apple had $252.3 billion of cash and investments held outside the U.S. in order to avoid stiff repatriation taxes. That probably increased to at least $270 billion by the end of December, as Apple usually produces very strong free cash flow during its first fiscal quarter.

Under the new tax reform law, Apple will incur a one-time repatriation tax of 15.5% on its overseas cash and investments, less than half of what it might have owed otherwise. Assuming conservatively that Apple had $270 billion held outside the U.S. at the end of last quarter, this tax bill would come to just shy of $42 billion, leaving Apple with an extra $228 billion to use for buybacks, dividends, acquisitions, research, or any other purpose.

Apple will be able to repatriate huge amounts of cash to the U.S. this year. Image source: Apple.

Most analysts assume (quite reasonably, in my opinion) that Apple will set aside a bunch of its overseas cash to pay down debt, while using most of the rest to buy back its stock. Milunovich gets to his $122 billion estimate by assuming that the company will produce about $60 billion of free cash flow annually and maintain net cash of $90 billion.

This level of buybacks wouldn't be that unusual

This $122 billion might seem like a huge number. However, it doesn't look quite as impressive when compared to Apple's buyback activity over the past five years.

In total, Apple has repurchased $166 billion of stock during this period. Furthermore, looking just at the two-year period beginning in April 2013, the company spent $78 billion on buybacks. Since its stock price was much lower at this time, $78 billion was enough to reduce the share count by 818 million (on a split-adjusted basis). To buy back 818 million shares at today's stock price, Apple would need to spend upwards of $140 billion.

Thus, repurchasing $122 billion of stock within a two-year span might itself be unprecedented, but that sum wouldn't buy as much Apple stock as the company repurchased during its most productive two-year period for share buybacks.