I need an equation to calculate the time for a power percentage of a half cycle sine wave into a resistive load
If you really want power, which you probably do, you want v^2, since for most loads, power is usually proportional to v^2. 

v(t) = (sin(t))^2

Integrating yields:
t/2 - sin(2*t)/4

Evaluating from 0 to t is the same, since sin(0) = 0.

To get a percentage, you need to divide by the value at 100%.