i read somewhere that it takes almost a month for them to give birth (from conception) [about 5-6 weeks]. as for signs i don't know.
I do know that it's not healthy to leave them in a small container for more than a few days when they a preggers. It could cause stress that could cause birth defects.
If it is time for her to have them, gradually raise the temp a few degress (3, 4 or 5 degrees). If she don't give birth after 24 hrs, put her back in the main tank.