score:2
There is no time required to keep them. Throw them away whenever you want.
It is better to still keep them digitally, even in an old email.
Some uses for keeping them:
If you do bin them, dispose responsibly to not reveal any private information. Even your barcode could contain information about you.
If there is no electronic access whatsoever even online with the reference, i recommend keeping them for a few months and then it's your choice if you want to keep them for memories/other uses.