I replaced the belt on my Sport City 250 at 11,500 miles. Some signs of wear and very small cracks. It probably had a few thousand miles left on it but the recommended interval is 12,500. The mechanic at the Aprilia dealer said that he did see a Sport City with a failed belt. It had around 18,000 miles on it. The BV350 has a lot more power but I would hope it has a larger belt as well.
I ride, Therefore I Am.