I would buy new kits yearly, and keep them stored in a cool dry place (like you would medications or something).
You can still use up the old kit, but compare it to the new one periodically to make sure it's still reading accurately.
If they're stored in a very hot environment or an environment that fluctuates wildly they will go bad quicker. I had some that accidentally got stuck in my attic and when I found them they didn't test right, despite the fact that they should have still been good if stored properly.
Do not leave ammonia absorbing media in more than a couple of days. If you leave it too long you can starve out your biobacteria, then when it's depleted end up with an ammonia spike, just like if you suddenly added a bunch of new fish. That said, I don't know about the effectiveness of the particular media you mentioned on nitrates, ammonia, or nitrite, since I've never used it.
You can also detox nitrates with prime, and though they'll still read on a test they won't be harmful to your fish. No need for expensive media.
Another option for nitrates would be to run an aquaponics system on your tank. Use plants that heavily use nitrates, such as ones that grow fruits/veggies (a lot of ppl use tomatoes). Usually even high nitrate levels can't keep up with the needs of such plants.
Even if your tapwater is 20ppm, you can still get your tank below 40ppm which is safe for most fish. You just have to do larger water changes and more often. 20% per week is nothing compared to what a lot of us do, which is 50-90% weekly, or more (discus keepers often do large 80-90% changes every couple of days)
You could also consider doing a drip system, where you allow a small amount of water to drip from your tap into your tank (you must use a chlorine filter inline for chlorinated water), and excess(old) water comes out an overflow in tank or sump. Even with just a seemingly negligible amount of water dripping into the tank 24/7, it is very effective at reducing nitrates and other pollutants. There should be directions on how to make such a system in the DIY area of the forum.
! I decided that whatever I'm doing so far is not going to cut it (unless I do a 100%wc, which I wouldnt dare do). Therefore, I went to marinedepot which is only 10miles from my area and purchased 1L of Seachem De-Nitrate and Brightwell XPort NO3 300G media. I put ONLY the XPort NO3 in a filter bag and put it in my filter. I will add the De-Nitrate for additional help but really for reducing nitrite and ammonia spike. Its been 2 hours now and did a water test. Previously the nitrate level was at 47ppm.....now (after 2 hours) its at 40ppm! Man this stuff really works. The professional told me that within 24 hours, the ppm will get to 0ppm and that once it starts to rise just replace the media. Man, I shouldve done this rather than do three consecutive wc and stress the crap out of my shoal... BRIGHTWELL!