With proper care and maintenance, wooden cutting boards can last a lifetime. That said, it's time to replace your cutting board if it has become noticeably warped (making for an unsafe cutting surface), cracked (bacteria harbors in nooks and crannies), or otherwise irretrievably damaged.
Our favorite wooden (and bamboo) cutting boards:
John Boos Square Reversible Maple Cutting Board ($186),
Totally Bamboo Cutting Board ($35, originally $60)