Dear Dragoons. Your AoE is actually good.

[StouterTaru]

False information. A ninja using Huton (as every good Ninja should) will have a much shorter GCD than 2.5. Huton is a 15% attack speed increase, which means their gcd gets 15% shorter. Assuming 0 sks, as is ideal (though also not possible), a Ninja has a 2.12 gcd. Using this number, I made this table.

The table assumes you just missed the 3 second TP tick when you used the first DB. Also, it leaves out the time of casting 2 Doton. I used 2.1 sec GCD and rounded up to get 40 sec.