All of the autos I have done started to flower @ 3 weeks old, though I know some wait until 5 weeks old. The finish line is very different for each strain, one taking only 56 days to finish while another may take 120+ days.
If it is not in flower now (at least starting), it is not an auto and will take 12/12s to flower.
Bag seed can be fun, you never know what your going to get.