How long does leftovers keep in the refrigerator?

Official USDA FoodKeeper guidance for one exact storage condition

RefrigeratorGeneral storage guidance

3 to 4 days

This is storage guidance from the cited FoodKeeper record, not a guarantee that food remains safe through the final day.

Refrigerator at or below 40 °F (4 °C)

Food: Leftovers
Preparation or package state: without meat, fish, poultry, or egg (such as cooked vegitables, rice, or potatoes)
Storage: Refrigerator
Timing: General storage guidance
Source field: Refrigerate

Source: USDA FSIS FoodKeeper Data, data version 128.