DOING IT TOO TIGHT
If you twist when the new growth is too short, a couple of things can happen; The hairs, if too short, won't allow enough space for the Loc to be looped through if using the interloc method with the rug-latch. This is unnecessarily painful and can cause hair to pop right out the follicle never growing back resulting in thin roots. Sometimes the new growth is sufficient but the loctician may use the NSEW method and this often results in breakage in an attempt to force the thicker loc through a smaller space when trying to loop it through more than once. This is also one of the leading reasons that people get turned off of the interloc method; for fear of pain and breakage. Twisting new growth and loose hairs too tightly in order to keep a manicured look can also lead to similar results as well as receeding your hairline. As with most things, too much of anything isn't good for you.
WAITING TO LONG
On the other scale of things, waiting to long can bear similar results as over tightening. Neglecting to tighten your Locs for too long can cause undue pressure at the line of demarcation, which is where your new growth meets the already formed Loc. The already formed Loc is far more dense than the root, and the root which is made up of our loose hairs, are still curls that will stretch out when pulled. As a result, the already formed lock weighs down and stretches out the curls in the new growth. After a while, the line of demarcation gets weak from the unbalanced force of gravity. Think of tying a rock to the end of a rubber band, and stretching it further and further..... eventually, you know what happens. And it looks a lot like this picture here.
SO WHEN DO I DO IT?
It is important to remember that the length of the new growth is more important than the time that has passed. Often caught up in rules and rituals, we forget that Locs are like fingerprints in the sense that they are truly unique to each individual. And even with that out of the equation, everyone's hair and nails do not grow at the same rate. Therefore it is unreasonable to expect your new growth to be the same as mine since we have different metabolisms, hair textures and diets.
RULE OF THUMB... PUN INTENDED
I learned a long time ago the importance of knowing the measurements of random parts of your body. Something to do with being creative.... With that being said, the top of your thumb is in an around an inch. If you hold your Loc at the root with the tip of your thumb touching your scalp, and the line of demarcation is past the top of your thumb, its time for a tightening. If a reference to time makes you feel better, every 4-8wks is a fair measure.