I have about 400 miles on my Maxxis Aspen EXO’s. They feel a little more loose than when I first bought them. Tread isn’t terribly worn down but I don’t really know how to gauge this (no penny test like with cars).
How do you know if you’ve reached the lifespan of the tire?
Personally I use Maxxis DHF, Shorty, HR2 and Ardents. Oh and I’ve currently got a DHF and Aggressor combo for summer fun.
Anyway…I bin mine when the nobs, in your case side knobs, no longer have any square edges or when they’re beginning to crack/come off. I don’t mean nice and fresh square edges but when the knobs are significantly rounded rather than having corners anymore 
I’d say that when that extra “looseness” starts bothering you.
For me, I generally replace tires when either I get a flat that doesn’t want to seal with a plug, or I have an A race coming up. Those sharp corners on the knobs are the key to the best grip from the tire. Once those are rounded, the grip seems to stay pretty consistent for quite a while until knobs start ripping off or really wearing down. Ii had a racing ralph on the back of my hardtail for like 2+ years. I was mostly using that bike for gravel & the more the tire wore out, the faster it rolled. I think it had 6 plugs in it when I finally replaced it.
Definitely location dependent as rocks beat up tires more than dirt and roots. So as others have said, there is no time or miles, but rather appearance and performance that determines replacement.