I got rid of the Phantoms on my Street Twin at 4,800. Still had plenty of tread but were scary in the wet with the traction control kicking in regularly. Changed to Avon Storms and haven't looked back, get about 8-9k from a set and night and day better. I think the new Spirit has replaced the Storm and will be replacing with those in the future. (Storms were a Triumph option on the Street Cup)
I run on the Deauville tyres that are harding wearing centre and softer on the walls and get a lot more kms out of them bit more expensive but i am getting old and dont have to drag the pegs anymore
Monitoring the tyre tread depth is obviously important all year round, but don't forget to swap out your summer air and put winter air in the tyres during the cold weather seasons.
Bit late to the party here. Depending on which bike, I usually run my tyres close to the 1mm minimum on the rear. I normally get away with the front for two rear tyres. Mind when I had the thunderbird lt I swapped both the front and the rear at the same time.