I'll give you this, covering both hooklink and hair length;
My hook links are usually between 15-20 centimetres long, 6-9inches as most of the lakes and rivers I fish are pretty weed free, and I use running leads, which may mean I can get away with shorter rigs than if using semi-fixed leads.
My hair length is always changed dependant on where I am hooking (or losing) the fish.
I start off with the hair leaving the shank of the hook directly opposite the barb of the hook, and the hookbait about 5millimetres below the bend, hanging freely, not touching it.
If I start losing fish, or hooking them at the very edge of the mouth, then I lengthen the hair.
If I am hooking them deeply in the mouth, or can see the hook has scraped down inside before finding hold, I shorten the hair.
If the hook is dead centre of the bottom lip, hair length is spot perfect.
Some additionals for you though;
If you feed more particles, groundbait or pellets, a big bed of free bait, then you may need a shorter hair and hooklink than if you feed just boilies.
In other words, how you feed may also contribute to how long a hair or hooklink you use.
If you fish in weed, then you may need a shorter hair to keep the bait tight to the hook.
Finally, pop-ups are a different kettle of fish altogether, and in most cases, should i use a pop-up, then the bait is on an extremely short hair, more shank mounted than on a hair at all.