I have a 50 gal for my two females. They are both full grown, and almost 4 years old.
I moved them to the 50 gal when they were still young, under a year, and they weren't full grown at the time. Before that I had them in a 20 too, and before that, when they where eggs/tadpoles, I had them in a 10 gal.
Anyways I think you can upgrade whenever you want. You don't really have to wait until they reach a certain size or anything. If it where me, I'd upgrade sooner rather than later just to give them the extra space as they continue to grow.
Right now mine are just in a regular run of the mill glass 50 gal aquarium, but I'm planning on switching my tanks over to acrylic in the near future.
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