MMO is just mineral oil and white spirits, and in that order. I have yet to hear of any negatives coming out of running MMO long term besides pre-existing leaks getting worse, purely because it will thin out your oil and these people in question didn't compensate for it - Marvel even warns about this on the bottle, if I recall. Mind you, you should be addressing any significant oil leak already before you go adding snake oils to fix other problems. I can personally say I ran it constantly on my last truck and never had any bad come from it.
In regards to time it'll probably take a couple full oil change intervals before you notice any difference, but that's preferred - better to slowly dissolve it then go fast and hard, knock a chunk loose and get it stuck between a rocker and lifter.
You could also look into other additives like Seafoam or Gumout, that are advertised as being oil additives (on top of fuel additives) and contain things like naphtha or PEA, which works great against carbon and cooked oil. Or, as previously mentioned, BG EPR. Never tried it myself but people have praised it for decades.