I might be able to finish Mass Effect 1 by tomorrow guys. I've just gotten into the Archive on Ilos and I'm calling it a night.... at 3 a.m. Go Dazzle. 
Hah, I remember playing ME3 through the entire night the first time... I just fell right in so damn hard.
Then, at 7AM, I managed to pick wrong and murder off the Quarian Fleet. And make Tali kill herself.
I never nope'd out of a choice so hard. Actually, I don't think I nope'd out of any choice before (or after) that...
Yeah. If you're there, you're in the home stretch... 
Yeah. I just hope she finished everything in the Galaxy she wanted to.
There's a straight jump into ME2... of sorts.
No post-completion gameplay. You just get the Normandy flying near a planet, in a scene eerily reminiscent of ME2's beginning scene...
So for all intents and purposes, there is zero time jump in-universe between the two games.
Well, the first one, and second one's prologue, anyway. Not gonna spoiler stuff.