Inconsistent chain play--causes?

[RSK70]

Not to hijack the thread but in the process of changing my oil today, I spent a lot of time on the ground and noticed my chain is rusting a bit. I will try to get a picture or two in the morning, but when -- if there is even a general time -- should chains be replaced? I have an 05 with ~14k miles and thought it was odd to see some rust. The rear sprocket looks good still and honestly haven't sat down and look at the front 

I did test the chain play in probably the most unofficial way possible as I don't know what the right way is. I will scour the wiki soon but in the meantime I realized my chain can be picked up toward the seat substantially(2-3") but pushed down very little (~0.5"). Is this meaningful in any context? I still have a lottttt to learn but am very appreciative of the knowledge I've found on this site so far.