Hi!
It looks pretty good, nice fat stem, leaves look fine and the internodes aren't too far apart so it looks like you have your light low enough. Lucky genetics too, at least regarding that trait. Unless you're getting problems on several leaves, don't worry too much. Nutrient problems will show up on whole areas of the plant, like new growth, or lower leaves etc. One common early mistake is to read into every brown spot and irregularity on any individual leaf and go into high gear trying to treat it, which usually does more harm than good.
It looks like a good candidate for topping now, right above the 2nd node. That takes care of all of your height problems instantly since you're chopping it off. It'll grow back giving you 4 main colas, and give you lots more opportunity for training, lst, etc to deal with the vertical growth. Plus you can take that nice, juicy, hormone-filled top cutting and make it into another clone.
If you're only doing that one plant and have no horizontal space problems, you can get a nice yield out of a 3 gal and toooooons of opportunity for LST all around the pot. Even consider topping after the 3rd node then, which will make it bushier and have plenty of bud sites, but maybe not such prominent colas.
For flowering, count on the plant increasing by 1.5-3x depending on your strain and even the individual phenotype of your strain. There really isn't a set time where you have to flower, it's all up to you. Do you want to let it get bushier? Are you LST'ing? Are the shoots 12" and you want them at 18"? And the most important thing...how long can you wait to harvest? I'd love to grow a 16 foot tree that's big around as my house with 1000 colas but I also want to enjoy the harvest sometime this year
