It's generally unwise to treat what you like as the discrete set "ballbusting." Saying you're into it applies a label to yourself which may suggest to her some things which aren't necessarily true, or otherwise cause her to categorize you as "one of those guys who's into that," rather than just a guy who happens to like some thing which fall into that set.

Better to simply say what you like... and if she thinks it's weird, you can tell her that a lot of guys like it, and that they call it ballbusting. Make that explanation third person, not first person... "there exists a fetish and I'd like to try some of it" is a lot less scary than "I have a fetish."

That assumes you haven't done much of it before, and that you enjoy vanilla sex. If you can't get off without having your balls busted, you might as well just come out and say you have the fetish.

Also... how long you take to ease her into it depends largely on her sexual maturity, not yours. If she's new to the whole sex thing, or has only ever done vanilla and doesn't know much about fetishes, take it nice and slow. If she's done some freaky shit before and doesn't think anything of it, just say it upfront and save yourself the trouble.