As a hobbyist, all you'll really need to do is store it in a cool, dry place, and it will last you for years (beginning to end of spool). Keeping the wire closed over from tinning keeps the flux core cut off from air also helps.
![Smiley :)](https://www.eevblog.com/forum/Smileys/default/xsmiley.gif.pagespeed.ic.R8GFI-pF6f.png)
Worst case, if you find it's not performing satisfactorily, just cut off a couple of feet and see if that doesn't improve matters (cutting back to fresh, unexposed flux).
![Cool 8)](https://www.eevblog.com/forum/Smileys/default/xcool.gif.pagespeed.ic.cOxV6sPNRd.png)
So go ahead and buy the 1lb roll.
![ThumbsUp :-+](https://www.eevblog.com/forum/Smileys/default/icon_smile_thumbsup.gif.pagespeed.ce._JElyJQqdB.gif)
FWIW, I've got a small tube that's ~30yrs old (RadioShack), followed by some Kester that's over 20yrs old that I'm still using.
I also keep liquid RA flux on hand for use in a refillable brush pen (
MG Chemicals Rosin 835 comes in hobbyist friendly sizes). Cheaper than disposable pens.