Depends on your latitude, I'd expect them to show around 20 sept-1 oct @45N.
Angola Thai started flowering at that time, also Malawi...and they would finish in mid-end december, maybe even later in january.
Most plants show preflowers at the 5th or 6th node. The light and soil make a difference in preflowers so I don't know when it will happen. Start counting where the first set of true leaves start growing. That is number 1 and you keep counting until you reach the top of the plant. A magnifying glass is almost necessary and the true sign of a female is two white hairs peeking out between the main stem and where the leaf grows out. It makes you crazy waiting for something to happen but you will be OK when they finally show.
I'm glad to see that you finally managed to produce seeds of your own with the Purple Haze Thai male ... the crosses will be long flowering, but very interesting for the sativa lovers. Please, keep us updated with the outcome of the hybrids. Best wishes!