Transferring old metal particle Hi8 tapes
You do need to play the tapes onto a capture card, yes. And the sooner the better. You don't just get one attempt, but you will get a slight degradation of quality every play, so you are best off aiming to do it in one pass if possible.

I recently captured about 50 tapes from the late 80's to the mid 90's and I did about half of them then cleaned the head then did the other half. Degradation wasn't noticeable, but I just thought I should do it just in case.

As to how long you can store them - is the safe place shielded from EMF, heat and humidity, then they can last for years, but due to the nature of magnetic tape, the magnetic field strength of particles affects adjacent particles, so quality will degrade no matter what.

When configuring your capture, set it for the highest quality you can afford - as there are utilities which can clean up some noise and speckle once the signal is digitised.