If I read correctly, Jack Daniels suggests the following for a workout of the given type:
Tempo (5k pace + :25/mile) - up to 10% of weekly mileage, (with very short recoveries if you're doing cruise intervals)
Interval (5k pace) - up to 8% of weekly mileage w/1:1 work:rest ratio
Repetitions (1500m pace) - up to 5% of weekly mileage w/1:3-4 work:rest ratio
If his reputation alone isn't enough to provide validity to these guidelines, I can only offer limited anecdotal support in the form that these guidelines worked very well for my team this past season. We had runners with vastly different abilities and bases (20 mpw - 60 mpw), but still managed to get everyone to improve significantly, peak at the end of the season all the while avoiding injuries.
Best of luck to you!