I used to ride the Anakee 2s until they were flat in the middle on the rear before changing, you could feel the edge when you got it leant over. The fronts I changed before it got to 1mm, I could usually tell when it was wet that they're about to need a change. I'd say 1.5-2mm is still safe, lower than that and braking in the wet starts to suffer enough for it not to be worth waiting longer to put a fresh one on. Not much of an issue in the dry though, but in the UK it can get wet at any time.