There is no single rule that applies to all butter products when it comes to determining how long you can leave it out of the refrigerator before use.

Food spoilage bacteria need water to grow and when left out of the refrigerator, butter can become contaminated and unsafe to consume. However, butter is a water in oil emulsion, which means the fat globules structurally wrap around water molecules and make them harder for bacteria to access.  

This offers some protection against food spoilage compared to other dairy products and makes it safe to soften butter at room temperature over a period of time. You would not want to consume other dairy products that have not been refrigerated for a similar time. 

There are a number of factors which would influence the length of time you could safely have butter softening in an unrefrigerated setting, including the fat and salt content. In addition, butter is very susceptible to absorbing other flavours and odours. The key reason it becomes unsafe to eat is rancidity because of fat oxidation, which produces off flavours. This process is accelerated if the product is exposed to oxygen, light, and elevated temperatures. 

When it comes to softening butter, the best practice is to do so in a controlled setting where it cannot be impacted by cross contamination, and it is protected from exposure to oxygen and light, or elevated temperatures. Users should talk to the supplier/manufacturer of the product for specific advice.