On average, hair grows 1/3 to 1/2 an inch per month so you should get 4 to 6 inches of growth every year.
Protective styling involves using styles that protect and hide your ends from damage so that you retain more length.
How long can you expect your hair to continue growing for?
On average 4 to 7 years. Everyone has a different growth cycle. This means that if your growth cycle is 4 years and your hair grows at 4 inches a year your maximum potential is 16 inches. This means that you cannot aspire to have more than 16 inches of growth ever. With trims this should be about 13 to 14 inches.
Now, some people have a really short growth cycle of 1 year, 4 to 7 years is just average and the lucky few have a growth cycle of upto 10 years. Those long-haired people that make it to the Guinness book of world records probably fall into the latter category. Even with 4 inches per year, 10 years of growth is 40 inches of hair - wowza!
If you feel like your hair is not growing it simply means your hair is not retaining length, that is, it is breaking off at the ends or you have reached your maximum possible.
So if you have a 1 year cycle at 4 inches per annum then 4 inches is your maximum potential growth, genetically. This is very rare, you probably have a longer cycle.
Will your hair grow longer if you use protective styling rather than if you don't?
Yes and no.
If you use protective styles and all your other hair care practices are poor then your hair won't grow to its maximum potential. For instance if you keep braids in for far too long and forget to wash and moisturise them regularly. Or if you use excessive amounts of heat and have an atrocious diet then your hair will suffer.
However, if you take care of hair and make smart use of protective styles then you will retain more length