I stored some electronics in self storage for a month and a half one summer. Be sure to keep anything valuable off the floor in case of a leak or anything like that. Also, I would cover all your stuff with plastic drop cloths used for painting. These will help keep dust off your stuff, and will provide some protection against water. I wouldn't store anything too valuable in one of these units (for instance I wouldn't put my main PC in one). You might consider putting some of your smaller stuff into plastic storage tubs (such as a VCR, DVD, CD player, component stereo system) if you are really worried.
Ryan