Big Brother typically runs for about three months, but the exact duration varies by season. If you’re asking about a specific season labeled “Big Brother 20,” here are the likely durations based on the US version:

• Big Brother 20 (US) aired in 2018 and lasted 99 days from June 27 to September 26, 2018. The finale crowned Kaycee Clark as the winner. [source: season page summaries]
• Other countries’ “Big Brother 20” seasons have different lengths (for example, UK series 20 ran 42 days, the shortest regular run in its history). Always check the country and year to be sure. [source: series overviews]

If you meant a different country or a specific season, tell me which version (US, UK, Australia, etc.) and I’ll give you the exact start and end dates for that season.