Determining how long any given wig will last is attributed to multiple factors. Generally, if a wig is taken care of, meaning worn through the day, taken off and stored at night, washed and maintained properly in regular intervals, they will last on the average 6 - 12 months. Some wigs will last less than that, anywhere from 3 - 6 months. The brand, hair fiber, whether synthetic, human hair, heat friendly hair all are factors.
Becoming accustom to wearing and caring for wigs tends to become as natural a part of your lifestyle as if you own hair. The life-cycle with needing it professionally cut or tailored to your needs upon first purchasing it, to washing, conditioning and styling as necessary, will become familiar to you. So, take good care of your wig, treat it with respect, you invested in it, make it last as long as you can. You'll know when it time for a new one.