Hair extensions usually last for a few months. Two to three months. I wouldn't go any longer than three month. You must also keep in mind that when you do take your extensions out you will have a lot of shedding. Hair sheds normally 1000 strands a day. So for three month that's 30,000 a month, 90,000 strands in three months. That sounds like a lot, and its going to look like a lot when you comb it out in three months at one time. It looks like less when you comb your hair out everyday and its only those 1000 strands.
