I haven't been able to find anything definative on how long air is "good" in a tank. There is nothing to indicate it will "go bad" by just sitting in a tank.
However we have heard of problems with air after long term storage and know of a diver that becam ill after breathing air that had been in a tank for 2 years. It is still uncertain if it was the air that caused his problems or other factors.
We recommend that you do not use air over one year old. Since all tanks need a visual inspection every year (and we have to dump the air anyway) this should be an easy guideline to follow.
If anyone has any other information on this subject please forward it to Ken.
