I don't pretend to know how long the shelf life of green ethylene glycol antifreeze/coolants, is, but I've developed a healthier respect for these despite their being laced with silicates and phosphates purportedly hard on water pump seals and bearings. A friend asked me to change her green Prestone coolant out the other day. I asked her how long it had been. She said, "Oh, I dunno - 3 or 4 years; maybe 5." It drained clear, clean and with no jelled glop in the bottom tank to retard flow. I flushed with water until it drained absolutely colorless in a clean drain pan. I just put Wally*World ST green anitifreeze/coolant concentrate in as replacement finally diluted to 50/50. '89 Toyota pickup with 171,000+ miles on the original water pump. I told her not to let it go more than 2 years/24,000 mile this time. We'll see whether my pearls of wisdom went in one ear and out the other...