at least to my knowledge (which is limited in this area), there is very little science behind what is too long--probably more science, though, on what is not long enough, if you're looking for the specific adaptations that come from long runs. most practices are anecdotal, such as increasing weekly mileage no more than 10%, or the long run being 20% of weekly volume. this doesn't mean they're not good ideas, or that they're not sound practices, but there's very little science to support them. when i was training as a miler in my younger days with santa monica track club, our longest recovery day was 7 miles, but my longest runs were 14-16 miles (and i would sometimes come back in the evening for a 4-6 miler).
there really shouldn't be anything automatically wrong with a long run being double your 2nd longest run (a fairly arbitrary formula at best)--ultimately your body/health/desire/enthusiasm should dictate how long your long run is, not some weird math, and so many of us are different in this regard i would caution listening to anyone who tries to give advice that is too specific or rigid...
that's my story and i'm sticking to it,
cush