Hi all! long time lurker, first time poster here.
when should a bike lock be replaced/ when is it justifiable to buy a new one?

I'm asking in part because my 5-year-old kryptonite NY mini is starting to be a little loose. Like i can wiggle it quite a bit.

How long do your expensive locks last?

It's time to get a new lock when you think that thieves will target your bike, or when it becomes structurally unsafe

If the wiggle made it noisy when I was riding I'd change it, but there might be a way to add a little gasket to it to stop that. I bypass that problem entirely by how I carry my lock though.

I don't know how long my lock will last. I've chosen this lock rather than another one and other less secure one because I want it to last a lifetime. But i suspect i know how my lock will die as i got an idea of it last year when the tumbling system stopped working properly once by lack of lubricant. I repaired it pretty easily by adding some lubricant. Only time will tell how and when this lock will die but other than the locking mechanism it seems pretty much timeproof.

U should replace it right away, as soon as u suspect that it might not defend well, unless your bike is a beater.

I'm still using the same U-lock I bought in 1977.

When you are down to just one Key for the old one.

I've got an OnGuard U-lock that's been sitting outside locked to a rack at work 24/7 for about five years now. A couple of years ago it was starting to bind, but I sprayed some oil in there and it's been fine since.

A U-lock is mostly a deterrent. If someone has the tools to break an old and weak U-lock those same tools would probably break a brand new U-lock a few seconds later. I wouldn't worry about it.

https://en.wikipedia.org/wiki/Obsolescence

These days you can get replacement keys for a lot of locks, I only had one left and I bought a couple keys from kryptonite using the serial code that came along with the original set of keys. Much cheaper than buying a new lock.