Occasions when Confucius used standard pronunciation: when reciting the Book of Songs and the Book of Documents, and when carrying out ritual ceremonies. On all these occasions, he used standard pronunciation.
In Confucius’s time “standard pronunciation” was based on a court language that originated from the Western Zhou period (1046–771 BCE). Not surprisingly virtually nobody understood it, and it was only used by a small elite when conducting ritual ceremonies and reciting ancient poems and texts in much the same way that the church carried out services in Latin during the Middle Ages.
In everyday life, Confucius probably spoke in the local dialect of his home state of Lu, though his command of the more archaic standard pronunciation must have come in handy when he visited nearby states during his period of exile – all of which would have spoken vastly different and at times mutually unintelligible dialects.
These days, the standard pronunciation of Chinese is of course Mandarin, which is almost universally understood and spoken (to varying degrees of fidelity) throughout the country. Thankfully, China’s numerous local dialects have also continued to thrive alongside it.