According the the Yamaha Zuma 125 owners manual, it's recommended to replace the drive belt every 1200 miles. Depending on your riding style and performance upgrades, you may have to do so sooner! Have a spare one or replace it before it breaks!
Polini Performance Aramid Drive Belt (810x22x28) for Yamaha Zuma 125
This long lasting, heavy duty performance Aramid (Kevlar) belt is designed to have less flex than a regular rubber belt.