If you have males and females mixed, or if they were mixed in the store- yes they are most likely pregnant. Could be 1 week, could be 3 weeks. It's hard to say.
Careful with putting them in breeder boxes though. They tend to stress them out and do more harm than good. If you want babies to survive, but lots of plants in the tank (moss is especially good), and make sure they can't be sucked into your filter (put a sponge over it if they could).
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