"How many pounds have you lost?"
When people shout that question in a room full of people (yes that has happened), I usually just chuckle and say "Oh, it's a lot" and then they get the hint. When someone asks me privately and just wants to celebrate for me in a way... I tell them. If someone is just being nosy, again I'll answer vaguely!
For more info on my journey & goals, visit my blog at http://flirtybythirty.wordpress.com