I've put it back in the plastic juice jugs a couple of times. It's fine for short term storage, but you get a cardboard like flavor after a few months.
No, nothing wrong with staring at your carboy. Just don't start singing to it, or the neighbors might call the cops...
I use plastic fermentation buckets. It does leave some aroma behind. You can get rid of most of that by scrubbing them down with baking soda.
I've got a batch I pitched onto the yeast cake going now. This is a fairly fresh yeast cake, the batch only stayed in primary for two weeks. When I transferred it off I pitched the new batch on top of the yeast cake. I probably wouldn't do that a second time. I'd be concerned about it starting to taste like soy sauce. Wine does that sometimes if you leave it on the yeast to long.
It varies massively. Anywhere from about three days to several months. Suspended yeast cells in the liquid, temperature, yeast strain, etc... They all change things. It's best to fill a plastic soda bottle when you do the rest. When that is hard to the touch, your bottles should be carbed.