I would wait till you see your first amber trichome in a week or so, and then flush and nothing but water until she finishes out... but that is how I do it and there really arent any rules about this.
Strain dependant primarily on these timings. You can slow down blooming some by doing some things poorly...and you can force a quick end by doing things wrong.
But if all is well and the plant is having an optimal bloom it is strain dependant how long until finish. That and each person has their opinion of what done is. But take that person's opinion as right and it is still... strain dependent.
Some strains are done in 10 weeks. I like the ones that take closer to 15. Sux but is what it is.