Same here; some where around day 600 in my campaign and am not really enjoying playing anymore. Allied with the Southern Empire and have been at war with the Asari since day 450 I think (maybe earlier) and nothing has happened apart from capturing one city and one castle. I've also noticed that the AI armies just bum-rush the nearest AI army and get slaughtered. Every time I command an army myself, I attempt to keep casualties to a bare minimum, but it is of no use as the AI always comes back with multiple armies. TBH, I really think TW needs to implement population system that is depleted over time which would help stave off endless wars and make strategic battles all the more important. IMHO, if you face an enemy army that is 600-1k strong and you completely annihilate it, than it should take time for the AI to recover and vice versa; granted I know a lot of players probably would not like that mechanic at all.
Edit: This is with the 1.1.0 beta branch, not the main branch.