How much do you think the beta stage of a product / service should last?

Many companies create products / services and report the start of its open beta testing. This is probably normal - but what time do you consider reasonable for this stage? Half a year, five years? What are they determined by?

Considering that any project in the development process is changing under the influence of user comments, changes in the market situation, demand for certain functions - is there really a need to declare the beta stage? What does this give?