<p>The problem with my Anet E12 should be on USB connection itself.</p>
<ol>
<li>If powered down, the board still gets power from USB</li>
<li>If board is resett, USB will still connect</li>
<li>Still works on SD; seem like most part not damage.
What cause the problem? Where to check first? How to fix it?</li>
</ol>
<p>Note: This printer worked with no problem before.</p>

Your math looks correct, and is also a good approximation for what I've seen in the first few weeks with my own Ender 3.
Another way to calculate (to check yourself) is to calculate the volume extruded (nozzle area times extrusion percentage times print speed -- be sure you convert everything to the same units!) in a given second, multiply by the density of your filament (common PLA runs about 1.2 g/cm^3), and get a rough figure for how long it takes to print a kilogram of filament.  Your actual print time will always be higher than this approximation, because there are moves during which the extruder isn't running, infill is often set to lower extrusion level, and of course there's setup and cleanup time to account for.