Speaking from personal experience I can say it's not very often! On AA it's been a total of five times for me. I have had four short-haul upgrades (FAT - PHX, PHX - FAT, LAX - FAT, LAS - LAX) and one long-haul (LHR - LAX). On CX it's been two: one was SIN - HKG and the other LAX - HKG. I would like to think that all airlines would favour their own elites over and above those from other airlines in the same alliance when it comes to upgrades. It's also important to emphasise that different airlines have different business models when it comes to upgrades. With AA you have a mix of complimentary upgrades based on status, as well as upgrades using miles and a copay. JAL allows you to bid for upgrades.