As a rule, if your gloves become torn or contaminated, then it’s time to immediately switch to a new pair after washing your hands. You’ll also want to ‘clean and swap’ if you are switching from handling raw meats to handling fruits or veggies.
As far as what qualifies as contamination, use your best judgment, but things like coughing or sneezing, taking a smoke break, and going from raw to cooked foods – all of these applications and are definitely times to clean and switch.