This is what wikipedia has defined as the Regency era. I have always found wikipedia to be a reliable source.
The Regency era in the United Kingdom is the period between 1811—when King George III was deemed unfit to rule and his son, the Prince of Wales, ruled as his proxy as Prince Regent—and 1820, when the Prince Regent became George IV on the death of his father.
The term Regency era sometimes refers to a more extended time frame than the decade of the formal Regency. The period between 1795 and 1837 (the latter part of the reign of George III and the reigns of his sons George IV, as Prince Regent and King, and William IV) was characterized by distinctive trends in British architecture, literature, fashions, politics, and culture. If Regency era is being used to describe the transition between Georgian and Victorian eras, the focus is on the pre-Victorian period from 1811, when the formal Regency began, through 1837 when Queen Victoria succeeded William IV.
So it is considered to be between 1811 to 1820. Unless you are like me and opt to use the extended time frame from 1795 to 1837. Shh, don't tell the mean ladies who like to comment on Amazon.