I agree that there's no specific number. To my mind, a frequent flyer is one who flies noticeably more often that most of the people s/he hangs out with. A town police officer in my area - they tend to be incredibly great guys and gals, but as a group they're not big travelers - could be a frequent flyer with far fewer miles/trips per year than a top troubleshooter for Cisco Systems. It's all relative.