The S8+ lasts a bit longer than the S7 Edge with a slightly smaller battery. I assume the S8 will last a bit longer than the S7 with the same size battery, but won't reach the S7 Edge.
On paper, the 835 is 40% more efficient than last year's. But your screen is easily half of your consumption, so it won't be a 40% savings across the board. The display may be 10% more efficient, but the screen has grown ~15% from the S7. So that's a wash at best. So you're looking at a 20% increase in battery life, tops, based purely on CPU related functions. The gsmarena test shows the S8+ is a bit over 9% improved over the S7 Edge, but with a 3% smaller battery. So the S8 will probably be 10% better than the S7.