i agree with the other comments on genes vs alleles and that it is often more complicated than recessive/dominant as in u/atomfullerene's example with blood types. but if i'm understanding what you meant, i think the question you were getting at is can a rare/generally considered detrimental allele become beneficial and widespread in a population.

if that's what you were asking, the answer is yes. and in terms of time, it would depend on how beneficial the allele is. an example is sickle cell anemia. Whilst it's normally a rare and detrimental mutation, it became pretty widespread in africa because it can be protective against malaria. In this situation the death from malaria was more frequent than the death from sickle cell, at least before the child-bearing ages, so this change was decently fast. here's a link if you're interested in reading more about this: https://sickle-cell.com/clinical/malaria