Well, if the person asking you when you’re getting married is your mom, she may be pressuring you, and this advice won’t apply.. but assuming it’s casual friends who ask, my experience has been that people ask questions like that just to be friendly and conversational, really. They generally won’t much care what you say in response, except that if you seem offended or get rude, they’ll probably be a bit put off.
So I’d recommend just laughing and saying something like “oh, when I finally get this guy whipped into shape”, maybe with a wink to your boyfriend so that all parties to the conversation know you’re just lightheartedly joking. And then move on to the next topic of conversation. :)
If your boyfriend isn’t present, maybe go for something similarly lighthearted, like “eh, one of these days when I’m sitting around bored with nothing better to do”.
The point being, the question probably isn’t meant seriously, so give a similarly inconsequential response..