No worries as long as the lids and inner seals are tightly and cleanly fitted. The only Tamiya paint that I’ve ever had “go bad” was paint in jars that weren’t tightly sealed and which dried out.
New, from the factory and never opened - essentially has no expiration date. Consider how long the paint may have sat racked up on the shelf at the hobby shop. It’ll last just as long on your shelf or paint drawer.
When you open the jars / bottles, be sure to wipe off the screw cap threads and the plastic seal inside the cap (at least around its outer edge). This is really important especially if you just shake the jars to mix them. When you open the jar, paint will run all over the threads in the cap and the jar. Wipe all this off before closing the jar. If you don’t, the built up, dried paint will eventually compromise the air-tight seal and the paint will dry out. (Not to mention how hard it becomes to unscrew the lids. Keep a pair of Channel Lok pliers handy at your bench to help un-screw stuck paint jar lids if this is your M.O.)
Using a battery powered paint stirrer / mixed and draw the paint out using pipettes or medicine droppers will significantly mitigate this problem of paint on the cap threads. Still, take a moment to wipe them off if you do get any paint on the threads.
Never add water or paint thinned with water back into the jars. Tamiya paints are cellulose thinned acrylics - not water-based acrylics. However, Tamiya paints reduced (i.e. “thinned”) with proprietary Tamiya thinners are Ok to add back into the jar and don’t seem to have any effect on shelf life. (This does, of course, effect the thinning ratios later.)
IF you’ve allowed a jar to start to dry out and the paint has become very “thick,” use Tamiya lacquer thinner (“yellow cap” thinner) to thin it back to a usable viscosity. Mix very, very well with something that will reach all the way to the bottom of the jar - don’t just rely on shaking the jar to re-mix. A battery powered paint mixer is the best tool that I’ve found for this. (Re-clean the cap threads on the jar and in the cap before replacing it.)
Once the solids in the paint have started to polymerize and have turned into a semi-solid, rubbery clump in the bottom of the jar, it’s all over. Even if you successfully break the clump up, the paint will never be usable. The acrylic binders have done their thing on a chemical basis, and they’re done.
Finally, save a few of your old and empty Tamiya paint jars. Clean them spotless (hardware store lacquer thinner) and scrape the labels off. The inner seals in the screw caps will come out for cleaning. Avoid scratching or gauging the seal, though.
If you ever need to mix up a large batch of paint - pre-thinned for spraying or some custom color mix - as long as you stick to Tamiya brand paint and thinners and keep it in this clean jar, it’ll have a shelf life that’s nearly as good as a brand-new jar of paint. (I use blank peal-and-stick envelope labels, the kind sold in 8-1/2 x 11 to run through home printers to make new labels for these custom paints. They wrap right around the jars and stick almost as good as the original labels.)