If an exotic breed is used for the up-gradation of a local breed, how long would it take to fix the exotic inheritance level at 75% or 66%?
Your quetion does not have a simple answer. Are you trying to add a simple recessive trait? Add a dominant trait? Or trying to make the entire genome 75% of the exotic. If you are trying to make the entire genome the exotic, it could take only 2 generations. If you are trying to fix a single recessive trait, that too could be done in two generations. Trying to fix a dominant trait would take 2-3 generations since you would have to progeny test.
At least，1 generation for cross and 1-5 generations for backcross and 1-3 generation selfing for traits homozygosis. It is much depending on the trait, the gene under the trait and the genetic relationship between the exotic breed and the local breed.
it depends on heritability of a trait, species,relationship of the breeds