I haven't played this game in years and just picked it back up the other day. I remember that I only have many years to beat it in game. I just don't remember how many years though. Sense I tend to over grind wanna know how long I got.
I don't think there is one. I remember people claiming it was 5 years for Hard mode, but I've gone over 5 and the game didn't end.
Supposedly it's 15 years for Easy mode and 10 for Normal, but I don't play on those modes, so I don't know. I've never gone to 10 years on Hard, so I couldn't say for sure on that either. You could just put some strong knights on your borders than skip turns for awhile to see if the game ends after 10-15 years.