I've had mine for over a year, and I swap out the heads every two months because I brush pretty vigorously which, yeah the bristles start looking worn around then, and I don't want to risk not cleaning properly. Works fine for me which, yeah if yours are fraying already after six months, maybe check your technique; lighter pressure might make them last longer.
Also, storing it properly helps; I keep mine in a dry spot to avoid bacteria buildup. That way, you're not replacing more often than needed and saving some cash.
