Pre/Flowering Phases

When I go online to check the flowering period of some of my strains, and it says, for example, that it's a 10 week strain. Does that mean that it is ready to harvest around 10 weeks from when I switch the light cycle, or do I have to wait for the preflowering to be done. Does it also matter that most of them were preflowering slightly in the vegetative phase?
And when flushing, should I give it one or two weeks for soil? I use Fox Farm nutrients.

10 weeks from the start of 12/12 if plants is mature, 10 weeks from when the plant first show sex if flowered before mature. it is good they were preflowering, it means they are mature and are ready to start flowering when you are. Flushing is a very debated topic. i wouldnt do longer than a week flush.