Will the clay layer be problematic and How long sh...

If you can grow tomatoes in your soil you can grow weed they are very comparable in terms of growth habits. Your in a very dry area so too much rain rotting your roots shouldn't be an issue. Clay conditions make it difficult for the roots to penetrate it creating a shallow root system which can be prone to heating up and drying out too quickly. Since your grow looks like it's in great shape you can lightly top dress your soil with some good quality organic soil mix and then apply a thin layer (3")of organic mulch to help keep the root zone cooler and conserve moisture. Just pull back the mulch when you feed then recover the soil. As far as harvest time. Go by the buds. The white pistils will begin to dry and turn orange then curl into the calyx. When about 50% of the pistils have dried check the trichomes with a 40x jeweler loupe with led light(Amazon.com) Depending on what type of high you like will determine when to chop. The trichomes will start off mostly clear then turn cloudy/milky then amber you will have a mix of all of them on the buds. Most folks like to chop when they have 5% to 30% amber trichomes. You can also harvest in stages to try and see which you like best after drying and properly curing your bud. You have a way to go by the looks of the bud development. I hope this helps you out.👍