I've gone 400 turns so far without first contact. I seem to be in such a corner that the AI's may never find me, unless I go out after them.
The definition of full exploitation that requires you to develop all the colonization techs and convert all the atmosheres would tend to make that even longer.
However, I see that as a matter of personal choice. I enjoy the game like this, but those looking for a lot of conflict would not. Don't forget the long term promise of challenging conflict, when you make first contact & discover you are WAY behind
