Mostly with new rats, it's a waiting game. You're waiting for the curiosity to outweigh the nervousness, it takes longer with some rats than others, but with rats, curiousity always gets the best of them in the end.
Taking treats outside the cage though may or may not ever happen. 4 of my 5 rats takes treats wherever they are and have from day one, Boris my hairless rat however never takes treats outside of the cage. He's just too busy to be concerned with the food when there's all that sniffing, exploring and climbing to do.