Many people think that a wig will last for years, when in reality the lifespan of a wig is much shorter.
A good quality human hair wig can last a few years if properly cared for and not worn every day. If you have two or three human hair wigs and rotate them every week or so you will get the most life out of them.
Quality and proper care are the main factors that determine the lifespan when it comes to human hair.
Synthetic wigs are a whole different story. Most manufacturers recommend between two to four per year. Short synthetic wigs will last longer than long ones. Longer wig fibers will tangle and become matted fairly quickly, requiring frequent replacement ( 3-4 a year).
With all synthetic wigs the main factor iS HEAT! Exposure to heat such as ovens, hair dryers, curling irons, etc. is the biggest cause of damage and therefor the need for replacement.
Even if you are careful about heat exposure, synthetic wigs wear out fairly quickly. Having more than one and alternating them will help some, but be prepared to buy two or three a year if worn frequently and want to look good consistently !
Hope this helps, John