The life of a human hair wig is basically dependent on two essential factors, the quality of the original wig and how carefully the wig is looked after.
The finest quality human hair will be remy and unprocessed, which means that the hairs will have been kept aligned in the same direction to minimise tangling and that no additional treatments such as colouring or silicone additives (to make the hair feel smooth) have been used.
The hair will be knotted carefully into a lace cap a few strands at a time and then sealed with a light adhesive to prevent shedding. The more coarse lace wigs will last longer than those made with a more delicate fine lace.
The wig will be kept on wig head when not in use, or stored for longer periods in an airtight container at normal room temperature. It will be washed with salt free products to prevent the hair drying out and allowed to dry naturally.
If all these things are minded you could expect a lifetime of over 2 years and possibly longer: however its possible to destroy a human hair wig in under a week by using unsuitable products and washing and straightening at extreme temperatures.