It could indicate the beginning of a nute deficiency, or it could be the strain. Some guys have two different phenos of the same strain, one with perfectly normal petioles (the stem for the leaves) one with entirely purple. If your plants are showing other signs of deficiency I would look at the nutes. Otherwise I wouldn't worry.
As to how far along in flower you are, a few weeks?
Does it really matter? They're done when they're done and unless you're looking at setting up a perpetual grow with some clones the timing will vary.