I just reached 25,000 km on one of these tires, all of it rear wheel loaded touring use. The puncture protection belt is peeking through, but only in sections and not yet along the entire circumference. I'm guessing it will last till 30,000 km.
These tires are heavy as hell, so you'd better make sure you don't mind because once you buy it you're going to be riding it for the rest of your life.
And they're not immune to punctures. I've had a handful of flats on this tire, all from metal wires.
How long has your Schwalbe Marathon tire lasted?
Let me get this straight, the puncture protection belt is showing and you want to ride it another 5,000k (3,000 miles)? ARE YOU INSANE? LOL!!
You need to buy another tire, either stay with the same tire you have if you like that tire, or get a lighter weight Schwalbe Almotion tire. I only have about 2,000 miles on mine but I've had no flats after running over miles of glass and who knows what else, and there are no cuts on them, the tires still look new. Supposedly the Almotion tire is supposed to last 8,000 to 12,000 miles loaded riding from reports from other riders. I decided on the going with the lighter weight tire after having a pair of Marathon Greenguards that were harsh riding, and felt sluggish, the Almotion tires have the least amount of rolling resistance of any touring tire, and it's noticeable even under load compared to the Greenguards. The Almotion has two different flat protection belt options, V-Guard and RaceGuard, the RaceGuard is the toughest one against flats. However, mine are the older EVO TLE version that used what is now called the V-Guard, so I have the lighter version and still never got a flat or even a cut so far.
I did cheat with those Almotion. The RaceGuard weighs about 150 grams more than the V-Guard, so I wanted to make sure these tires wouldn't go flat while doing a loaded tour, so I installed a pair of Clear Motion Rhinodillos flat liners that weigh 130 grams which brought me to as much weight as the RaceGuard, but the RhinoDillo is actually better at flat protection than what the RaceGuard would have been, and the Rhinos are far better than Mr Tuffy plus they're lighter in weight. I tested an old Mr Tuffy that I had and the Rhinos with a tack, and the tack penetrated the Mr Tuffy easily, but it would not penetrate Rhino; plus with a pair of scissors I could cut the Tuffy like cutting butter, while trying to cut the Rhino hurt my hand.
Now if you want the best touring tire, though it will weigh more than what you have now, the Marathon Tour Plus is a step up.