I have estimated the impact of a level buyback program in AAPL shares to be an increase in share price of about 3% (plus or minus a % error factor) per month, everything else being equal using a static analysis.
This may not be enough to offset the effect of other negative factors that can impact the price of AAPL stock from time to time.
The ultimate effect of the buyback on AAPL share price and volatility will depend on how the program is implemented (level or some other approach).
Further details from the Company would be helpful and would warrant disclosure to the investing public.
Disclosure: I am short AAPL.
Additional disclosure: My position in AAPL may vary from day to day.