The AI is generally well and thoroughly beaten before it actually surrenders. The exact "last straw" is therefor very difficult to pin down and usually is something quite arbitrary. Often times the AI surrenders when its fleet gets cornered by a force many times larger. This can feel frustrating if this was the final move in a hour-long game of cat-and-mouse that the AI likes to play when it's on the defensive.
Ultimately, the AI's surrender behavior is a tad unpredictable. Having a larger fleet is the best way to encourage it to happen. I did a bunch of reloads of a specific scenario once, and confirmed that the AI surrendered much sooner when I increased my fleet size, even if I wasn't actually using those units. The second largest factor is usually the AI's own empty fleet supply. If it has a massive amount of unused fleet supply and insufficient income with which to fill it, it will often surrender. In other words, conceding an economic victory. The exact number of casualties you need to inflict to do this is variable, so again it's hard to predict when it's going to happen.
The bottom line is that I've never seen the AI surrender when it has any real fight left in it, so don't worry too much about a premature finish.
