The archive in Google Photos just is a secundary stream stored at Google for photos you don't want to see in the mainstream. So as long as you don't start deleting or ruïning your account, they will stay eternally - just like your mainstream. Also see photos.google.com in the Chrome-app or in the browser on your laptop.

Nevertheless it always is wise to have an additional storage place for your original files on phone and moving your files to an sd-card is the worst option, because that tiny card may get lost or stolen together with your phone on some bad day.

So it is more wise to copy the new files to your computer via the usb-cable weekly or monthly.