My current game is coming dreadfully close to the end--and I'm around turn 500.  It's been extremely fun, even with incredibly powerful monsters that levelled like crazy prior to the 1.07 update (I killed them all, but it was *hard* and fun).  Even though one night I had to quit because I was mentally exhausted by them...

I consider turns 250 to 350 to be mid-game, but that's based entirely on my gaming style.  My Sov didn't even get married until around turn 150!  (Patience on that really paid off!!!)

Ignoring that fact that it is easy to win game quickly, I would consider 219 turns to be mid to late game.  Late game implies high level technology, which I would say is after turn 300 or so.  It depends on map size (and pacing?) though.

I said "probably" because I wasn't certain if it was a bug, or if offering a marriage that would not produce heirs annoyed the king enough to allow the option for war. lol

Then I must certainly be doing something wrong... I played a game up to turn 387 yesterday, I had three cities, hadn't even met the other factions, was wandering around looting all the resources and items that appear on the map with the Adventuring technology branch... The it started crashing and I couldn't continue on at all.

It all depends on how you play.  300 is mid-game for me.

I don't think you can really make a turn by turn judgment of what start/mid/end game happens. A lot of it varies based on location and happenstance. I think it can also vary from player to play within a game. I think at best, we can use loose definitions.

Start Game: Initial exploration, aggressive and/or relatively unhindered expansion

Early Mid Game: Expansion slows down, increased monster activity relative to size, consolidation of internal terrain and resources.

Late Mid Game: General borders and kingdom size more or less static, and general victory options are possible to consider.

End Game: General power rankings solidify, paths to victory are solidified, and ultimate fates happen.

Try limiting your strats and victory conditions to avoid the easy options.  I found the game lasted longer and was a lot more fun if I didn't use Sand Golems or any summoning spells until late in the game for example.  Hopefully the balance and AI issues will be fixed one day so it isn't necessary

My personnal problem is that i find late game boring,so i rush mastery quest with 2 squads of warhammers soliers + usually the quest dragon to end the game.Hunting faction isnt that fun.

Well it's my personnal opinions.