Final Fantasy 7 Remake - Where does it end?

The remake ends at the point where you leave Midgar, which happens very much like it does in the original. There are however quite significant story changes towards the end, including a final boss (or 2) that wasnt in the original. Which makes sense considering its the end of a full game and needs to have a climax or some sort.

Regarding how much of the "complete remake" this comprises... who knows. Part 1 shows how much the are willing to change or "pad out", so they could really drag this out if they wanted to.

The remake does not cover half, or even one third, of the original story. According to IGN:

only about 5-6 hours into what is generally a 40-50 hour RPG.

That means Remake covers about 10-15% of the full game.