Vaccine manufacturers say 2nd dose within 3-6 weeks:

https://www.cdc.gov/coronavirus/2019-ncov/vaccines/second-shot.html

Canadian experts: 4 months is good too

https://www.msn.com/en-ca/news/canada/vaccine-panel-confirms-choice-to-delay-doses-up-to-4-months-in-canada/ar-BB1fouYd?li=AAggXBV