how often should i put new line on my reel using 12 pound mono trilene(sp)? thanks

I never had a problem with leaving line on a reel (stored out of the elements) for long amounts of time. I change when the line is down to a point where it affects casting.

Some people aren't comfortable with this. If you're one of them, then by all means change whenever you start losing confidence in the line you have on your spool.

But I've had line on a reel for years with no problems. Of course, these are reels that I use for a certain application just a couple of times a year, otherwise the line would've been used up faster, but just the amount of time hadn't been a factor.

My biggest fish so far was a 35 lb. carp on 8 pound test that had been on the reel for several years.  My fishing equipment has been sitting idle for 25 years.  Just went and tried to beak line on one of them.  Nope.  I wouldn't worry about it.   

having your reel in the sun with line on it on a all dayer can make it loose its full potential pretty fast

I have had cheap line in the sun all day and within 2-3 months it was brittle and I have had quality line out in the sun for several days at a time and work great for 5+ years. So just check for problems, kinks, wear spots, or any problems.

Some people replace it every few months but to me that is just a waste. Considering line sits in warehouses and store shelves for months or sometimes years before you use it, a few months use will not hurt it.

If it's mono I make sure to troll the line out, I've had mono on spare spools for the longest time with no adverse effects.

I change lines as needed, when my spool gets low.  But, one thing I do is keep lures on my rods/reels to keep a little tension in the fishing line.  That way next spring when I go to use it, the line does not have as much memory in the 10' near the lure.