Well I don't think in kg's
so let me just say that bench tends to be more difficult to get going again than things like squat or deads. I can't predict what will work for you but the bottom line is you need some recovery. With dropping the weight I would look for at least 3 workouts before you come back to the current weight. For me that would usually be 5 pound increments.
Now that may be overkill or it may not. But look at it this way. You don't want to drop the weight back too far but if you don't drop it far enough and end up stalling out again really soon, that is more time wasted than if you had just dropped it a bit further in the first place.
And of course this is not the only choice. You could bring it down to sets of 3 for instance.