i have daily backup instances to cold standby qemu-kvm VMs with snapshots of qcow2 storage which are kept for 10 days and i skip deleting every weekly Monday backup until space becomes a problem (usually months). sometimes the qcow2 storage backend becomes too slow with so many snapshots and that's when i qemu-img convert it which basically fixes it but removes the snapshots, so i keep both of them for a while.