Smallpox does not mutate much if at all. The 30% death rate refers to the fatality rate in someone with no immunity (ie: previous disease or a vaccination {which fades with time}), this assumes no modern medical care. Things like quarantine only help slow the spread, they don't affect the outcome once you get sick.
It's hard to say how long it would take for the Native American population to end up with the same level of natural (as opposed to acquired) resistance to smallpox, but it would be quite a few generations at least (given ~20 yrs/per generation). Between the disease itself and the dislocation of farming and hunting, children left without parents etc, I would expect the Native American population to be reduced by at least 50%. Given it will be quite some time before "American" resistance reaches European levels, rebuilding the population will be quite slow. And, as mentioned, there will be periodic epidemics which will be worse than they would be Europe.
It should be pointed out that smallpox was not the only killer imported from Europe. Influenza and tuberculosis both were imported, and as with smallpox the native population had neither inherent resistance or acquired resistance. What this means, even if smallpox comes to the new world 200-300 years earlier and spreads, the indigenes will still be vulnerable to some other serious imported disease. In Native Americans, TB was not a lingering disease but one that progressed rapidly.