I haven't been able to get it to work in-game at all, even on the longest straights no matter how close to the car in front.
And F1 cars do benefit from slip-streaming lol? That's pretty much the technique any f1 driver will use on a straight to overtake or get alongside his opponent. Don't know what F1 you're watching.
Some maths for you,
For some distance behind a high drag car, such as an open wheeled Formula car, the drag of a following car is reduced. This can be of the order of 30% reduction at 25 metres distance, and depends on the aerodynamic details of the two cars. This reduction in drag means that less power is needed to maintain the top speed and enables the car to accelerate (around 0.2g for a 450bhp car of the late �60's), even though it may be travelling at it's clean air top speed. If the driver accelerates from some 50 metres back behind the car in front, while in it's slipstream (drivers claim that they can feel the effect up to 100 metres behind), he will be travelling around 5 kph faster when he comes right up behind it. Popping out of the slipstream now enables him to use this speed advantage to overtake, but he will still use over 0.25 km to draw alongside and hopefully claim the next corner. Fast corners leading onto long straights were ideal for this manoeuvre.
So sometimes slip-stream effects are felt up to 100m behind. I'd, personally, say thats quite a distance.