Hair of any given type grows at a fixed speed.
How long it gets depends how thick each strand is to start with and how fast it's worn away. Each strand starts to wear as soon as it gets beyond the skin and the older it is (so further from the skin) the more wear it has experienced.
Each strand tapers, in other words, until it gets too thin or brittle and the end disintegrates.
(That is the source of the myth that cutting hair makes it stronger or thicker - if you cut part way along a tapered thing, the cut end is automatically thicker and stiffer than the original end was...)
The more you mess with it and remove natural oils, the quicker it wears and the more brittle it becomes.
That is why some people claim that some type of oil "makes hair grow better" - it actually reduces wear so the hair survives to a longer length.
The only other thing that influences it is general health - having a good, balanced diet with no fads or exclusions. Many people do not get all the nutrients they need, due to force of habit or circumstances.
A general multivitamin may help then. If your diet is lacking anything and you add a multivitamin, after a month to six weeks or so you will notice your fingernails get a slight ridge growing out - they are starting to grow at proper thickness rather than the cells being starved.
The same thing will be happening with every hair strand, but it will take many months or even years for the new stronger and thicker growth to reach full length (unless you have a crew cut :)
If you ever revert to a poor diet even for a few days, your hair and nails will go back to their thinner growth and a "weak point" will grow out after a while.