Is the game over after you finish the last part of this quest? The assassination.

I have been putting this off mainly because after doing it the NCR is hostile to me but also because I feel that it will be game over.

Doing Caesar's quest line what is the point of no return, or the river rubicon as Caesar would put it.

If this isn't the end, then about how much more is there?

After you assassinate Kimball and complete the quest, then Caesar will say, OK, we're ready for the final battle. u rdy 2 go d00d? It gives you a big warning screen saying END GAME THIS WAY. And then you can say yes and go do the final battle, which took me somewhere between 30-60 minutes. You can also say no, not yet, hold your forces, and then go do other stuff in the game for as long as you want.

So no, don't be afraid to finish this quest. You get plenty of warning before the end.