How long do the FEDS (not the state you live in) require you to keep info on guns you've bought and sold (without an FFL)?
Whether you need to or not. I would be interested to see all the different models you've had. Also some people are just record keepers. Would be easy to open a flash drive and note, manufacture and model # , whom you bought it from, serial#, and any circumstances when bought. Sometimes there are
funny stories. Flash drives take a few seconds to use then close and store where you want. Would be part of your history when your gone.
Are you doing this for profit and not just casually trading around? Then you need an FFL.
Are you doing this as a hobby to enjoy different firearms? Then you don't need an FFL.
If you don't need an FFL, you do not need any records of any kind. So "how long" doesn't matter, as you don't need them in the first place.