Rate of hair growth is predetermined genetically.But on an average hair grows at a rate about 0.45 mm per day that is 1.35 cm per month. Other than this, the factors that affect the rate of hair growth is nutrition , hormonal imbalances and environment. It is important to get checked for any vitamin deficiencies. This can be easily detected by a blood test and once you consult your doctor they will supplement you accordingly. Usually it takes about a few months for these nutrients to get back into the required levels in the blood. Environmental causes are pollution bad hair care routine and any kind of hair products you usually use and any trauma you caused to your hair by straightening or anything. You have to be very careful with you hair, especially in weathers where it is dry,.polluted and you have a lot of substances that stay for a long time on your hair .Washing your hair regularly, oiling it regularly and keeping your nutrition and lifestyle healthy will really make a difference to your hair. Do get a small trim for your hair every 4 to 5 months to prevent split ends because split ends slowdown the rate of growth.