She'll overtake Victoria, who reigned for 63 years and 216 days. Elizabeth II has currently ruled for 62 years and about 300 days.
She's already the oldest living monarch we've had, as Victoria died 7 years younger than the Queen - who's currently 88. So there's a good chance we'll witness the moment Elizabeth II becomes the longest reigning monarch of the United Kingdom.
God save the Queen.
In just under a year's time, ours will be the longest-reigning Queen