I have a new stradic 2500FI ready to go with  8# yo-zuri hybrid.  It came with a spare spool, so should I fill the spare up too?  The hybrid is supposed to last all season if I am correct.  How long can line stay fresh on an unused spool?  Should I switch them out every once in a while?

I have the 2500FI also.  I have one spool filled w/ 8 lb P-Line CXX & the other w/ Sufix 20 lb braid.

With all the spinning reels I have w/ spare spools, I have one w/ mono & the other spool w/ braid.

When I spool up w/ mono, I use Reel Magic & spray some one after so many windings.  So if I don't use the spool the line on the spool stays fairly soft & will manage easily when it's time to use it.  

Oh, I also using mono backing for both mono & braid.  I only spool enough of what I am gong to use, about have the spool or so.  So I have extra line when I need to respool later on.

How long can line stay fresh on an unused spool?

Years, if not exposed to sun and heat for long periods.

I have ultra soft on one spool and braid on my extra spool.

Spool up the spare and keep it with your tackle... it only takes one fish to wrap you and scar a full cast's worth of line and then you are using weak abraded line.  If it happens 2 or 3 times, you'll be out of line.  

Where we fish, we carry spare spools even for our BC reels and a spool of line to boot.  If for example, I'm t-rigging all morning, I often need to change line out over a sandwich at lunch time.  

Even if you never use it, a spool's worth of spare line is a couple dollars of well needed insurance, IMO.

I usually carry a couple of spare spools of line in the test poundages that I'm fishing, just in case I need it. In fact, last Saturday one of our party needed to put some line on one of his spare spinning outfits and I was able to accomodate that. Cheap insurance, IMO:o)

I like to have 2 spare spools for each spinning reel I bring. 1 with the # test I'm using on the other one. And the other one with a different # test. That way If conditions call for something different I can change it out easily. I usually don't change though. I like to find somthing that works and stick woth it. But like Popeye said it's cheap Insurance

Well thanks everyone for your input.  I spooled it up with the rest of #8 line just incase I hook into something big and he runs all the line off my reel.