There is no one definitive timeline for King Arthur, as his existence is the subject of much debate. However, some historians believe that if he did exist, he may have lived in the 5th or 6th century AD. This is based on the fact that the earliest written accounts of Arthur date from the 6th century, and that he is believed to have been a contemporary of the Anglo-Saxon invasions of Britain.
Other historians believe that Arthur may have lived in the 9th or 10th century AD, as this is when the majority of the Arthurian legends were written. However, there is no archaeological evidence to support this theory.
Ultimately, the timeline of King Arthur is a matter of conjecture, and it is likely that we will never know for sure when he lived, if he lived at all.