Alan,
The amount of training in the tempo zone depends upon how intense the effort is for you, first of all. Intensity and duration are inversely proportional. So, if you run faster, you can run as long, and vice versa is true also. The following is an estimation of workout stress for various paces within the tempo zone:
LT (one hour race velocity): 4 pts per mile
HMP (half marathon velocity): 3 pts per mile
MP (marathon velocity): 2 pts per mile
Let's use some examples and figure out what is equal.
If you run 4 miles at LT, you earn 16 points total.
If you run 5.33 miles at HMP, you earn 16 pts total.
If you run 8miles at MP, you earn 16 points total.
These are only estimations and can vary according to your race pace. But, it gives you a place to start.
Now, my idea of good use of tempo training is to get in 20 points per week. Tinman