I had the infamous "LeMans A4" Dunlops on the back of my Commode, the were AT LEAST 5 years old, coz the two previous owners drove like Grandmas. Apart from being useless when new, they ranged between comically Doriftastic in the dry, to scary-loose in the wet. After I'd been trying to kill them for a year, some uphill starts at the lights left me just sitting there, wheel spinning at idle.
I tortured them to death in the end, and by christ did I have fun grinding the tread off those bastards!!! So there ya go: 3-4 years for tyres to get super-dry & hard.