Now that I have a single stage RCBS and start sett up the bench. It occurred to me that everything comes whit some type of maintenance.

My question is how often are you supposed to clean reloading Dies?

Depends on how much you use them, whether you clean your cases before resizing. The short answer is when they appear dirty.

I imagine 'abrasive residue' and humidity are the greatest enemies of Dies. These likely dictate frequency of cleaning measures.

IMHO - Interior of Die is much more important than exterior appearance.

Hoppe's #9 on Q-Tip makes quick work of cleaning most Die interiors. Follow up with dry Q-Tip.

p.s. - Since I 'back-off' most Dies, one turn to neck-size only, not much of a concern.

Rarely on my part. 

I use an oiled Q-tip sometimes but if they start to show signs of binding on use, I use a pistol cleaning rod with a large patch on it that fits in the die just right. Any dependable cleaning fluid. Clean and dry well afterwards and lightly lubricate.

some full size dies I use graphite dry lube which works great! Can't mix with wet lube, though.