Necessary depth for land drain/french drain?

[Jeremy]

I've got a slope behind our house, which is just a few inches high at the lowest (East) point, rising to about 3-4' above the floor height in the house at the highest point (the  Westernmost side of the back wall). The soil behind is gathering water which comes down off the hillside and we're getting various kinds of seeping (sometimes more!) ingress through the bricks behind into the houe. The obvious solution to this is to trench out the soil along that wall outside and install a french drain. I have in mind to trench out about 12" wide so that I can install the drain about 6" away from the wall with a slight slope directing water off the house to be safe. The question is, how deep should I go? I *can* go all the way down to 4' at the highest part of the slope, but I'm wondering if this might be overkill? Do I just scrape off the gravel and dig down to the end of topsoil? Should I try to get to bottom of subsoil?