so this is a very basic question but I have scanned the MinnBankers Retention manual from 2017 as to how long a bank is required to save the night deposit log...I cant find anything other than chapter 3 page 8 which doesn't mention a period of time.
any help or resource would be appreciated

From my experience (15 years as Teller Supervisor in Minnesota), we threw the envelopes away immediately and kept the logs for one full rotating calendar year.

For example, at the end of February, I would shred the 2017 logs and replace with the 2019 logs.