Assuming no remaining patent issues, generics will start right away. However, as an incentive, one generic manufacturer will be granted exclusivity for six months where the price will not drop much. After that, the more popular the drug, the more competition and the price will drop considerably. But in the past ~10 years, generic prices have risen sharply. https://www.fda.gov/drugs/cder-small-business-industry-assistance-sbia/small-business-assistance-180-day-generic-drug-exclusivity