Biology question

So. Let’s talk about maggots. How fast do they grow?

We had a batch of about twenty or thirty of them squirming around the floor of the shower this morning, and I’m trying to figure out how they got there. They were about a quarter inch long, and very active. Rest of the bathroom seems clear; they didn’t come in through the fan vent as far as I can tell (at least, I wasn’t buried under the expected Flood O’ Maggots when I poked the vent open to check.) I suppose it’s possible they came up through the drain, but none of the other fixtures in the house have any unwanted guests.

Is it plausible that they hatched and grew there literally overnight? Or do I need to keep looking for entry points?