Studies and reviews are taking place. The NIHR one suggests prevalence of symptoms lasting over a month in the 10-20% range.[1]

As with any other early stage research relying heavily on self reported symptoms, the prevalence figures are not well established and the mechanisms causing it not yet understood, but they don't look like flu.

[1]https://www.bmj.com/content/371/bmj.m3981, https://evidence.nihr.ac.uk/wp-content/uploads/2020/10/Livin...