This seems to be an ongoing question for so many. Some people don't know how long they should hold on to such an investment or when to let it go. I am going to help you out abit.
First of all the one important detail that is outside of your hands or the technicians hands is the actual quality of the hair. No matter how well the technique is if the batch of hair is bad it is very little that can be done to salvage that weave. This doesn't happen often when you invest in quality remy grades of hair, however it is still possible (more on hair quality later).
Generally speaking a sew in weave should last anywhere between 1 and a half months to no more than 3 and 1 half months. This range is so wide because blended hair, human hair, and remy will all hold up differently. Consider this if you don't like to keep the same hair longer than 4 weeks or so, or you are trying something for an occasion why make such an investment on hair that will or should outlast the duration of time you plan
