Let s say we have some critical information stored as Zero Value transactions on the Tangle. How long does it last?
Which robust approaches do you recommend to keep a copy of those transactions?
I am using the testnet.
Nodes can choose to take a local snapshot at any time, which would delete old transactions from their ledgers.
Therefore, to be sure that your zero value transactions are not deleted, you would need to run your own node that does not take local snapshots.
See the developer documentation for information on running a node.