time. If your team is down 40-0 at half and is showing no heart, maybe that's an exception, but I don't think I would boo. Attendance, ticket sales, etc. are a better way to show your dissatisfaction as a fan.
That having been said, players should rarely, if ever, bash or talk bad about their own fans. It just shouldn't happen. CFB would not be possible without fans and you don't bite the hand that feeds you, literally.
That is all.