As there are 10 years in a decade and 365 days in a year, a decade will always have around 3650 days in it. However, the exact number depends on how many leap years the decade has. For instance, the current decade runs from January 1, 2010 to December 31, 2019. Two of the years in this decade, 2012 and 2016, were leap years, so this decade has exactly 3652 days in it.
The next decade will be the 2020s and will run from January 1, 2020 to December 31, 2029. There will be three leap years in the 2020s, namely 2020, 2024, and 2028. Therefore, there will be 3653 days in the next decade.