Jamie,
That's always a good question to ask. It's probably fair to say that when not in use, the typical bus suspension will leak down in as little as 2 days and as long as a week. Consequently, many coaches sit for long periods of time with flat airbags and it doesn't seem to effect them much. However, if you look at them when flat they look pretty bad so a number of us have been using Jon's custom built stands (blocks will work) to support them when in storage. Over time the stands probably make the airbags last longer.
As to how long they last, it varies depending on a number of factors but atmospheric conditions and time in storage are probably the main considerations. Like all rubber components they deteriorate with time. I personally start looking at them and at rubber brake lines with suspicion after 5 years.
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