From July 1 through July 31, 2014, simply convert your Starpoints® to American Airlines AAdvantage miles and earn a 20% bonus. As always, for every 20,000 Starpoints transferred within the same transaction, SPG® will automatically add another 5,000 miles. With the additional 20% mileage bonus offered by the AAdvantage program, you will receive a total of 30,000 miles from the transaction.
Starwood can post up to 79,999 points to a given airline mileage program during a 24 hour period. And of course you want to transfer in chunks of 20,000 to maximize your bonus.
So don’t transfer more than 60,000 Starpoints (which will yield 90,000 American miles) in a 24 hour period. If you want to make a larger transfer, break it up across more than one day.
Starpoint transfers are not ‘live’ and do not post instantly. In my experience they’ve sped up, however, such that it is generally possible to put an award on a 5 day hold with American AAdvantage, transfer the points, and have those points arrive in your account in time to issue the tickets. That didn’t used to reliably be the case, but I’m glad that it has consistently been so for me over the past couple of years.
There does not appear to be any limit to this bonus offer, outside of the theoretical limit to the number of points you can transfer in a 24 hour period and the number of days left before July ends.
(HT: murtuza in the comments)