In the majority of people hair grows approximately 1/2" per month or an average of 6" per year. Hair tends to grow faster in the summer and slower in the winter.
With that said, everyone has a genetic hair growth cycle. Some people can grow hair much faster than 1/2" per month, others may only grow hair at the rate of 1/8" per month.
In other cases, hair may not grow at all due to a dizzying array of potential issues ranging from nutritional deficiencies, dehydration, lack of sleep, excessive stress, reaction to prescription drugs, illness such as high/low blood sugar, hormonal imbalances....etc.,
There are some things which can help hair grow to it's genetically optimal growth cycle. However, if your hair typically grows 1/2" per month, that's usually the max it will grow.
Unfortunately there is a lot of misinformation on the Web regarding how to accelerate hair growth. Ultimately your hair will grow as fast as it is genetically programmed to grow.
Hope that helps.