Currently on an average week I hit about 18% of my mileage at marathon race pace or better. For example on an 80 mile week I get in 14 miles (8 mile tempo run at marathon race pace and 6 miles worth of intervals @ 10k race pace) at marathon race pace or better.
From what I have seen and experienced I do well when I keep this % between 10% and 20% of my mileage when training for longer races (i.e 10k through marathon).
I consider the 15%-18% range to be my sweet spot for more high quality training. Above this and I tend to run into over-training issues and below that I'm not maximizing the positive effects from training.
Have others found this to be true in their training? Are your ranges different? I have seen some comments on the subject that suggest 25% is a do not cross line, and I would agree, but little talk about a sweet spot.