Whatever FPS you'd get with an RTX2070 in a certain game at certain settings at 1080p, you're likely to get about 8-15% more with an RTX2080 [At 1080p]. So if you're getting 100 FPS in a game on an RTX2070, you'd expect to get around 108-115FPS on an RTX2080.
For more taxing games, that get closer to 60fps, chances are you will see gains closer to ~20% though, in that case it could be the difference between 50fps and 60fps, so it depends a lot on the games you play, look for reviews that have those in.