In Reply To:
Unless you are in the final build/peak stage for an A race, I think you'd get far more out of 3-4 x 1mile rather than running 400s.
3 min rest is too much between 400, as well. Cut down the rest to no more than 100% of your interval time.
My best estimate would be as someone else guessed: 18-20min, and probably toward the longer end.
A far better 400 workout for you at that pace would be something like 10-12 x 400 @1:30 on 2:00 to 2:15. The intervals will seem slow, especially at first, but rest assured...the final 3-4 will be painful, especially if you stay true to the shorter rest interval.
If you choose to run the miles, try something like 3-4 repeats, at 95-105% of your 5k pace per mile, and with 50% or less of the interval time for recovery.
Anyway, that's more than you asked for...hope it helps