I have some Armadillidium vulgare (fairly large species) I started culturing on a whim. It took maybe... four to five months for offspring to be born and begin reproducing, but original were all long dead at that point. My husbandry was a bit lacking at the start. >.>

After the second generation (~150) reproduced, though, the colony grew to easily the low thousands. Not a bad increase from the seven (I think?) individuals I collected last fall. Remember they don't have to be anywhere near full grown to reproduce, which helps a lot. I would imagine Porcellio scaber to be roughly similar to A. vulgare.