How to tell when you should be switching to flowering

I am just trying to figure out by looking at the plant itself how to tell when its time to take it into flowering, i have only had it a week but it came to me as a big girl not no tiny clone like i thought id be getting! ( which i would have liked to watch the plant all the way from growth but i took what i could) anyways so im not sure exactly how old it is or anything or how long its been in veg except the week i have had it. this was the other day. she looks alot more healthier then when i got her. and bigger as well
Ive dealt with a couple deficiencies .

Also, the guy who grows and harvests this plant has told me to beaware the leaves always look super stretchy, but has never effected his crop.

Some one respond.....

You can flower technically w/e you want by switching to 12/12, but I've read that plants do better if allowed to.grow to sexual maturity before flipping to flowering..
Two things I look for before flipping .. one is alternating nodes.. as a plant from seed grows, at first the branches from the main stem will come off evenly, meaning one from the same spot off the main stem, only on other sides of the plant. This will give the typical Xmas tree look to the plant ..
As the plant gets older, you will start to see what is called alternating nodes, which all it means is there will now be a lil bit of space between where near branches will come off of the main stem, nodes..
The second thing I look for is preflowers.. preflowers usually start around the 5th or so node on plants grown from seeds..
Both preflowers and alternating nodes are signs that the plant is reaching sexual maturity and is ready to be flowered @ any time..
One most also remember that a cannabis plant will usually at least double in size after the flip, possibly even triple or more depending whether it's more on the indica side or sativa side.. indices will stay shorter while sativas tend to stretch a bit more.. so if size is an issue in your grow area, you might want to flip a bit sooner, or preferably look into training techniques like LST or mainlining or super cropping to control heights..

Hope that helps some..