Well, however deep my last gf's was, maybe 1inch more would be great. I'm just average, but we still ended up cervix-boxing in some positions, especially when we got a little too enthusiastic. I agree though, tightness is much more important than depth... what good is a long tunnel if it's so loose you can't feel a thing? I'd rather only be able to put in 5inches of it but have it be nice and tight than stick the whole thing in but fall in after it!