Yellow low leaves- plant is using up its Nitrogen stores. Leave the leaves on and let them drop off naturally so the plant can get maximum access to the stored nutrients.

For all the 'how long to harvest' questions... Flowering time can be anywhere from 6-12 weeks depending on strain. So from the day that you found the females, 4-10 weeks or so. Since most people don't have patience to grow a 12 week flowering Thai, and 6 weeks is not common, I'm going to put my money on 6-7 weeks after you find the girls, then get out there with your scope and look at trichs until your eyes cross.