When using a slow cooker, the food doesn't need to come to a boil within two hours for it to be safe. Slow cookers are designed to cook food at low temperatures over an extended period, which allows the flavors to meld and the food to become tender.
The main concern with food safety in slow cookers is ensuring that the internal temperature of the food reaches a safe level to kill any harmful bacteria. The general guideline is to make sure the food reaches an internal temperature of at least 140°F (60°C) within the first two hours of cooking. This temperature is sufficient to prevent the growth of bacteria.
However, for certain types of food like meats or beans, it's better to ensure they reach an internal temperature of at least 165°F (74°C) within two hours, especially if they were previously at room temperature or chilled.
In your case, with beans in the slow cooker for over two hours but not coming to a boil, it's important to check the internal temperature of the beans. If the temperature is not yet at a safe level, you should turn up the heat to get them boiling or transfer them to another cooking method to ensure they reach the safe temperature zone as soon as possible.
Remember that cooking times can vary based on the size and type of the slow cooker, as well as the quantity and initial temperature of the ingredients. It's always best to use a food thermometer to check the internal temperature of the food to ensure it is safe to eat.