Hehe I've missed decent debate in here since the last one of these. Okay I'll give my idea for the timeline, most of it has evidence that would be too long and complicated to put in one post so if you wanna argue feel free.

Two thousand years before the game Jenova arrives on the planet, she manipulates and kills Cetra until finally an army confines her. She is trapped, with no way to escape.
Sephiroth is born and is given huge amounts of Jenova. Jenova posseses the ability to control her cells remotely, and this is her best chance. By the time any chance arises Sephiroth is too strong minded to be controlled, so Jenova waits.
Five years before the game, Sephiroth discovers his history and is driven insane. Finally his mind is weak enough for Jenova to take direct control. She makes an attempt to free herself, but Cloud stops her and she and her best chance at escape are once again inprisoned in some form.
By the time the game starts her body is spread over the world in various 'clones' (not a very accurate term IMO). She does however get another chance at freedom, Cloud. He is another with person with high Jenova levels and a very weak mind. Through chance, or possibly slight manipulation, Cloud winds up asleep in the same building as Jenova, she makes him free her.
Jenova still needs Cloud, for this reason and possibly others she takes on Sephiroth's image. She needs the rest of her body together to return to full strength however, and so the reunion begins. Sephiroth is the largest store of her missing body, and is trapped in the North, so she goes there.
The game progresses as is seen with Jenova dropping pieces of herself to slow, but not kill Cloud. These pieces then head north later.
Cloud breaks free of control in Mideel, Jenova loses all her power over him. He is the last remaining piece of Jenova, her strength is great enough anyway.
Jenova's pieces SYNTHESISe, after being defeated, she joins with her largest outside source, to become Bizarro, and then Safer.
Sephiroth is finally freed from Jenova but his mind is destroyed, he has nothing to do but fight.
I don't believe Jenova died after the Safer battle, if she dies, Holy kills her.