It Depends. I will say that I get upgraded to first class on domestic flights all the time, but have only gotten two or three international upgrades.

In Delta's case, at least, it depends on how many seats are free, and how you rank. The ranking is based on all kinds of stuff. So for less-flown and less-upgraded routes, it's easy to upgrade. Other routes have more people flying first class by paying and more competition for the free upgrades.