You should replace your vacuum's belt every 3 months?
Your brushroll belt provides agitation to clean your carpet and if you have a self-propelled vacuum, the self-propelled belt turns the wheels so you don't have to push.
After about three months, most belts stretch out causing them to slip. This means the brushroll turns slower or not at all and the self-propelled becomes increasingly harder to push.
Belts are inexpensive to replace and can save you hours of cleaning time and effort.
