I don't know when it's ever been changed. I bought the house in 2013. It has an insulating blanket and I don't really want to tear it up if I don't have to. Starndard 6=7 foot tall gas one.
Unless you want to replace your water heater more frequently then necessary, replace the anode every 3-6 years (depending on how hard your water is). Use magnesium for city water, aluminum for well water. If there is a ceiling above your water heater you should purchase a segmented anode rod (it doesn't matter if you bend and destroy the old one taking it out). Measure the length of the old anode after you remove it (if there is anything left) and cut the new one to the same length. Alternatively, you can drop a measuring tape through the opening and measure the length to the bottom of the tank, cut the anode an inch or two shorter than your measurement.Unless you are having an odor problem, I wouldn't mess with it.
I just use the rod I mentioned earlier for all my water heaters. Even for the ones where the ceiling is too low, I just cut the hot/cold water inlet and use shark bite fittings. The segmented anode rods don't have as much material and won't last as long as my .9 inch thick anode rod. I use shark bites mostly these days so a few of the water heaters already had them so didn't even have to pay for the shark bite fittings. The segmented rod will cost you close to $50 while the one I quoted earlier is just about $14. You don't need to drain all the hot water out, you just need to tip it a little to get the new rod in. Also the Milwaukee 2767 is 1400 foot pounds, I think most air guns are in the 400-600 range. I used to use a 3 foot pipe on top of an 18 inch breaker bar and two other guys holding the water tank to loosen the anode rod. Last time with the Milwaukee, it had no problems with 6 water heaters. Somewhat anticlimactic, a half minute later, it's loose. Just on 6 water heaters, it's already paid for itself.Unless you want to replace your water heater more frequently then necessary, replace the anode every 3-6 years (depending on how hard your water is). Use magnesium for city water, aluminum for well water. If there is a ceiling above your water heater you should purchase a segmented anode rod (it doesn't matter if you bend and destroy the old one taking it out). Measure the length of the old anode after you remove it (if there is anything left) and cut the new one to the same length. Alternatively, you can drop a measuring tape through the opening and measure the length to the bottom of the tank, cut the anode an inch or two shorter than your measurement.
You will need a 1/2" drive 1 1/16" 6-point impact socket. The thing can be a real bugger to get out. Mine was so tight that I had to use a breaker bar with a 5 foot cheater pipe, my air impact wrench wouldn't budge it. Don't drain the water heater before you attempt to take it out (but turn-off the water feed and relieve the water pressure on the hot side), you need the weight of the water in order to get that old anode loose, even then, I had to use a ratchet strap with a piece of wood wedged against the wall to keep the water heater from turning, it was THAT tight.
Eventually, when you have to replace the water heater, BEFORE you install the new water heater remove the anode, put some pipe sealant on the threads and reinstall it JUST SNUG ENOUGH to keep it from leaking (or have the plumber do it).
Segmented anode rods run between $20 and $40 on Amazon. At that price it isn't worth the hassle of disconnecting the water pipes and trying to tip the heavy water heater to get a solid anode rod in. Furthermore, even though they may have slightly less material, they are just as effective.I just use the rod I mentioned earlier for all my water heaters. Even for the ones where the ceiling is too low, I just cut the hot/cold water inlet and use shark bite fittings. The segmented anode rods don't have as much material and won't last as long as my .9 inch thick anode rod. I use shark bites mostly these days so a few of the water heaters already had them so didn't even have to pay for the shark bite fittings. The segmented rod will cost you close to $50 while the one I quoted earlier is just about $14. You don't need to drain all the hot water out, you just need to tip it a little to get the new rod in. Also the Milwaukee 2767 is 1400 foot pounds, I think most air guns are in the 400-600 range. I used to use a 3 foot pipe on top of an 18 inch breaker bar and two other guys holding the water tank to loosen the anode rod. Last time with the Milwaukee, it had no problems with 6 water heaters. Somewhat anticlimactic, a half minute later, it's loose. Just on 6 water heaters, it's already paid for itself.
I think I got about 9 years out of the last .9 inch thick rods I put in, I think those segmented rods won't even last the length of the original rods which is about 4-5 years. I'm not sure what factory is, they can be .5 inches all the way up to .9 inches. Of course the thicker the rod, the longer it lasts and the less often you have to change them. Looking at one segmented rod on Amazon, it's just .75 in diameter, some complaints are that it weighs less than the one they took out. I have 10+ water heaters so easier to just buy a bunch and use the same one for all of them. Doesn't actually take that long to cut the pipes and over half of them have shark bites already anyway so not even cutting them.Segmented anode rods run between $20 and $40 on Amazon. At that price it isn't worth the hassle of disconnecting the water pipes and trying to tip the heavy water heater to get a solid anode rod in. Furthermore, even though they may have slightly less material, they are just as effective.