When I bought my chest freezer, I went completely overboard and bought an enormous freezer (i can stuff an butchered entire cow in it, seriously and still have plently of room), my garage in the summer stays in the mid 80's and I don't notice much difference in electricity bills. While I am out there working, I will pay attention to how often it cycles and it does not cycle anymore than my indoor fridge. And it is practically empty, so its not maintaining the cold due to there being alot of frozen items.
So with that being said, I do not see it having to work much harder when ambient temps are in the 90's, unless it is horribly insulated.