How long do training memorandums for CBRN training need to be kept for?

I would keep anything past 2 yrs in a separate file for archive like in a separate storage area, if there's no more space in that storage area I would shred anything past 5... But I really don't like shredding unless it's past 10 if I coulof help it

What kind of training? CBRN defense team stuff, or general CBRN skills warrior task (level 10) general unit training? If there is any sort of specific task number associated with the training, then it should have been reported to the unit training NCO and entered into DTMS.

I would check with your BN CBRN NCO to see what he/she recommends. In many cases, that position is the one that should be conducting annual inspections to manage and monitor the battalion's CBRN readiness (including training). That person should know what they want to keep and should be in an SOP.

If you are acting without any decent guidance, I would recommend that you only keep about 2-3 years "active" in the book and move the rest to some sort of archive folder or file.