I have seen matings take place before the gravid coloration appears as well as after. If you witnessed a mating, 3 weeks after is a good guess for a laying date. She can have gravid colors and lay fertile or infertile eggs, or have no eggs at all. It is just a wait and see thing. If she is carrying eggs you will see the bulges in her sides as they grow. Most--but not all--females stop eating a few days before laying and start digging and looking anxious.
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Valerie

We may have all come on different ships, but we're in the same boat now.
--Martin Luther King, Jr.--

God bless the USA