I just changed the antifreeze in my JD2210 with the Cool Guard 50/50 pre-mix. Since my radiator only holds about 3 qts., I have enough left over to do about 4 or 5 more changes. Since my manual says to change every 2 years (antifreeze label says it's good for 5 years), it will be sitting a while. How long will the unused antifreeze last before going "bad"?