The solo player vs bot games should offer a surrender, say, when you've got more than 2x the total armies of all the other bots put together for multiple turns in a row.

I mean, after that point, it's just grinding it out, you'd have to be a moron to actually lose by then on ANY map.

You could vary the criteria if you had an exception on any map but for the most part the above suggestion seems obvious.

A HUMAN player might win, but not bots.