Most of the responses here are conjecture. A lot of theory, which is all true for very long term storage, but the timelines are unrealistic for what you're talking about. Assuming you actually use your shotgun a few times a year (which you d@mn well better be if you're using it as a self defense weaon), then you're doing enough maintenance to keep it in fine working order.

Oil seep, spring fatigue, case-head corrossion (voltaic cell created when moisture condenses in your action) are all incredibly long term effects. You're not talking about that length of time between handling and maintaining the weapon, so it's all irrelevant.

The 500's are like, well, we all know what they're like. Everybody has one, and they'll be fine as long as you keep it clean and take care of it... If you weaken your mag spring or mainspring by leaving it loaded and cocked, you'll find out about it in a hurry at the range when you're practicing with it, and you can get new parts in a heartbeat. I keep spare springs (among other parts) for most of my firearms, that way if I have a breakdown, I don't have to wait a week to get something shipped to me. Definitely worth the $5-10 to have a spare mainspring or mag spring laying around.

I paid $150 for the first Mossberg 500 I bought in 1998, been through half a dozen or so since then, paid $175 for a used one last week. Same ol same. Bought the new one to keep behind the seat of my farm truck, nothing fancy, but it'll work when I need it, and for sub-$200, I could care less how pretty it looks after a few years banging around in my feed truck. Mossberg 500's are the type of shotgun that I only end up getting in for work because someone has used them enough to wear something out. They don't wear out early, they just lend themselves to getting used a lot, because they flat work. (that said, not really a fan, but have a high level of respect for the 500).