When should cutting boards be replaced?
The USDA suggests that "all plastic and wooden cutting boards wear out over time. Once cutting boards become excessively worn or develop hard-to-clean grooves, they should be discarded."

Other advice they offer for avoiding food contamination from cutting boards: 


  Avoid Cross-Contamination The Meat and Poultry Hotline says that consumers may use wood or a nonporous surface for cutting raw meat and
  poultry. However, consider using one cutting board for fresh produce
  and bread and a separate one for raw meat, poultry, and seafood. This
  will prevent bacteria on a cutting board that is used for raw meat,
  poultry, or seafood from contaminating a food that requires no further
  cooking.
  
  Cleaning Cutting Boards To keep all cutting boards clean, the Hotline recommends washing them with hot, soapy water after each use;
  then rinse with clear water and air dry or pat dry with clean paper
  towels. Nonporous acrylic, plastic, or glass boards and solid wood
  boards can be washed in a dishwasher (laminated boards may crack and
  split).
  
  Both wooden and plastic cutting boards can be sanitized with a
  solution of 1 tablespoon of unscented, liquid chlorine bleach per
  gallon of water. Flood the surface with the bleach solution and allow
  it to stand for several minutes. Rinse with clear water and air dry or
  pat dry with clean paper towels.