It is really impossible to say. A modern F1 cars is completely built around the aerodinamics.
The loss depends on the type of the corners. In Monza it can actually be a benefit to follow a car, but if you follow a car over the whole lap, it can be a disadvantage even there. Generally the car loses so much downforce at the front, that the understeer gets tremendously huge and you feel the car won't turn in and you have to hesitate for a long while to get on the throttle.
Back to your question: I think you can lose two-three tenths in ONE single corner, but it all depends on many factors.