You can usually tell when the tds is creeping up - even my occasional assistant has commented on it and he runs a mortuary so I suppose he does have a scientific background!
As it creeps past 000 it starts to loose it's cutting power and is just cleaning by dissolving rather than consuming.
Still works, can leave streak free finishes as long as it's not in direct sunlight and does the job, it just takes longer, and time v tds is the determining factor in when I change my resin.