if hpta health is a major concern for you, then 6 weeks of test E would be the absolute minimum duration you could effectively cycle it. it is not optimal, as some bros will tell you...but again, if hpta health is your concern, then 6 weeks is the minimum.
i would highly suggest a healthy frontload of the enan, i'm talking something like 800 mg - 1.2 g the first week if you plan on using 400mg for the rest.
some bros will say that longer esters like enan and cyp should be run longer than 10-12 weeks for optimal results...which is true for the most part. however, the best enan cycle i ever ran was only 7 weeks followed by 3 of prop. it was structured like this....
week 1: 2 g enan
weeks 2: 1200 mg enan
weeks 3-7: 800 mg enan
weeks 8-10: 100 mg prop EOD
weeks 1-10: 1.25 mg letro EOD
pct on week 11: clomid, tribulus, zma
made excellent gains. PCT was a breeze. kept maybe 90% of what i gained.
good luck in whatever you choose.