There is a finite amount but isn't the reduction equation by half life, suggesting it's hard to eliminate completely?
Not sure myself - just curious. My understanding is that a 90 min moving boil with lid off should be adequate to ensure that DMS levels are under most perception thresholds but not that it will convert all available SMM.
I put my lid on at flameout to avoid anything falling in my wort (or a pigeon shitting in it, etc). At 80+ degree C, I probably don't need to be as cautious (it's about 80 when I run off into the NC cube after waiting for convection to settle and whirlpool etc).
One thing I have found interesting (somewhere in Fix, POBS, 2nd ed) that I have mentioned before is that some melanoidens can produce a flavour reminiscent of sweet, uncooked corn that some mistake for DMS (and is distinct from DMS which is slightly unpleasant, overcooked and vegetal). Interesting that a longer boil, designed to remove DMS may be creating a flavour confused with it.
Also interesting (to me anyway) is the large amounts of corn I get in bottled budějovick budvar. Not really sweet, uncooked and in levels beyond what i would prefer but not onion, garlic or overcooked broccoli either.