Kidder is spot on. Between 3K and 5K miles. A motorcycle engine needs mineral-based oil to burnish, i. e., seal the pores of the metallic parts of the engine, including the clutch. Synthetic oils, due to the properties that make them superior lubricators, don't perform this process well at all.
The burnishing process seals all the microscopic pores in your engine's metal parts, creating a naturally slick surface that's far more resistant to wear and has been shown to significantly extend engine life. Once that's occurred, then switching to synthetic will give you all the advantages synthic oils are known for.
Superbike racing teams have been following this same methodology for years with their track bikes. It's sort of like seasoning a cast iron skillet. No joke!