The guy that works the bike counter at my lbs (an outdoor supply company, tents, boots, etc...) says he changes his every 3 months, tires and tubes, just to be sure. "Your tires are looking a little squared off, I'd probably go ahead and replace them". They were Lithion 2s that had about 75 miles on them. He also doesn't trust patches. Ever.
I ride em until start having flats, or i see a cord. Move front to back, new tire on front, repeat ad infinitum.