you can begin to flower at the 4-5th node, are there preflowers yet? If not then they may continue vegging for a little while longer until they mature (I'm guessing these are from seed?) if they're clones then you can flower them at any point.
expect x2-3 their current length. Mine have only been in flower for a little over a week and they're beginning to put on their stretch. I doubt they will triple unless they surprise me. They may stretch more depending on the light they're under.