The certificate expires after 1 hour, so this is still an online process.
4: 1624458597, # QR code expiry 6: 1624285797} # QR code generated
1624458597 - 1624285797 = 172800 = 48 * 60 * 60
(I would have thought they could afford to be a bit more generous than that. If they were valid for a few weeks then it would be practicable to print them out.)
That said, the government communicated that it's currently valid for 6 month after the second jab.
This is because the length of effectiveness of the vaccination is not reliably known.
I don't know how it works with certificates, which are issued based on a negative test.