I swap out mine.... when I get yet another flat on the ones I have!
On my Jeeps, I don't know that I have ever ran a set of tires out of tread.
They fail in other ways (usually sidewalk punctures) well before tread life is an issue.
On my wife's vehicle though, I usually swap out when you can feel the wear, or you can feel slight slipping under wet road conditions.
As soon as she slips, it's new tires all around.
And when you get to know the guys at the Discount Tire around the corner by name... you know you're spending too much on tires.
3 personal vehicles.
2 work vehicles.
All with very high mile useage... and I buy a set of tires every 6-8 months.
'14 White Cherokee TH - Grand Canyon Brown
1.5" aussie lift, RRO step rails, rear hatch LED lighting, 31" KO2's, New transmission