Argon gas
Argon gas lasts in storage indefinitely. Keep the tank dry and clean. Protect it from being damaged by dropped items or being knocked over while in storage or in use. If not in use, leave the transport cap on the bottle. Never transport pressurized gases (argon or otherwise) without the transport cap on the bottle. If the bottle falls without the cap on, the valve may break off and the gas escaping at very high pressures will turn the bottle into a rocket, launching it with tremendous force. You wouldn't want to be in front of it, it can destroy most anything around. Think of it as a land torpedo.
Rusted tanks are very dangerous, do not refurbish them yourself. If the bottle is in good condition, most all of the unmixed gasses (oxygen, carbon dioxide, argon, etc) will remain usable for many years (though hopefully, you're using them up more often, otherwise there isn't much point in paying the lease and maintenance fees on the bottles). Helium mixes in tanks seem to be a bit problematic-the helium tends to separate if the bottle is left still for a while and seems to always find its way past the seals in the valve, leaving the mix in the bottle helium-poor. Given that you usually wouldn't have a helium mix on hand unless it was for something special, having a helium poor mix when you need the correct ratio can cause a delay while a fresh bottle of gas is found.
It's interesting to note that acetylene is not stored as a gas in the bottle. Instead, it is dissolved in acetone. In very old bottles, the dissolved gas is held in a layer of asbestos. Without this, an acetylene bottle would be EXTREMELY hazardous and would likely explode with a minor shock. Because the gas is dissolved in the acetone, extracting the gas too quickly from the bottle will carry acetone with it into the gas hoses (think of the gas coming out of solution like carbon dioxide comes out of a glass of soda) which can become a very serious hazard if undissolved gas is able to build up in the bottle. Never, ever, hit an acetylene bottle with a hammer or strike an arc on the bottle.
Acetylene is very unstable at pressures over 15 psi. That is the reason for the acetone in the tank. Acetylene cylinders are equipped with fusible metal relief devices (plugs) to relieve cylinder pressure when temperature greater than 212º F are reached. This protects the cylinder from over pressurization and possible rupture. Never attempt to plug a fusible plug leak by any means. Notify supplier immediately if the fuse plug is damaged or has deployed. Always treat compressed gas bottles with respect. Acetylene bottles doubly so.