Green’s Dictionary of Slang

a very long time.

1993		I. Welsh Trainspotting 122: It’s been likesay, donks, since ah went tae the fitba.
2012		(con. 1980s) I. Welsh Skagboys 57: Matty [...] has been skag-happy for donks.
2018		I. Welsh Dead Man’s Trousers 6: You’ve known though the grapevine [he] has been looking for ye for donks.