IMO:
1933 with the end of prohibition and 1961 with the closing of N KY illegals, 28 years.

Yes there were gambling joints prior to the end of prohibition but their main source of income was tax free booze. Gambling was an add on.

After 1933 gambling became the main source of income. Illegal casinos flourished in the era. Literally 1,000's of them.

Yes there were illegals after 1961 but they were/are few and far between after the era.