How long is a sew in weave supposed to last?

How long is a sew in human hair weave supposed to last? The average sew in, in my opinion, should last approximately 3 months. Here are a few reasons why:

Your real hair starts to smell- After being braided for so long, even though you may be shampooing the weave on a regular basis, you are not cleansing your real hair that well. Over time, the parts that aren't getting cleansed very well start to smell. By the three month period, you want to take it down and cleanse and condition your real hair properly.

Your braids can become matted- Depending on how well you take care of your hair while you have a sew in, leaving it in too long can cause your real hair to be matted.  Again, being braided for so long causes this.

You have lots of new growth- Due to the fact that your hair is in a protective style and no heat or elements of the weather are affecting it, it tends to grow much faster.  After three months you have around 1.5-2 inches of new growth.  This means that the sew in is not very secure anymore because the braid that the weave is attached to has grown out so much.