Technically, you can strand as far as you want, as long as you ‘catch’ the yarn somewhere in that long strand.
Also, I generally decide whether to catch the yarn in back based on the weight of the yarn and the actual length of the strand. If I’m knitting in a jumper weight yarn, which is very thin, I can let the strand go for more stitches than with a worsted or heavy weight yarn. Make sense? So I usually give it about 1/2 to 3/4 of an inch before I think the strand might be too long.
There is an excellent way to catch the yarn that doesn’t show through and doesn’t twist the yarn that I learned from the Philosopher’s Wool book.
Hopefully your teacher will show you how to do this or something similar when you take the class. It’s much easier to learn with illustrations or a demonstration, I think.
If you want to practice in the meantime, though, I’ll try to explain it in words.
Another important thing for stranding is to keep your stitches on the right needle stretched out to reduce the chance of puckering.