It really depends on a lot of factors (status, booking fare code, load for that flight, route, etc.). For me, I've been upgraded on less than 10% of my flights as a Diamond.

From what I've heard, it's easier to get upgraded on shorter intra-Asia routes, especially to more touristy and less business heavy destionations, i.e. places where there are likely fewer purchased C/F fares. This sort of meshes with my own experience, I would say.

In my case, frequently purchasing S fares and flying on long transcontinental routes probably works against me and together with my status (there are plenty of Diamonds on Asiana) explains the low upgrade rate.