He can hear the real question in there somewhere, and he smiles playfully at Watson, wondering at the answer. If Watson had announced his inclinations it's very possible Holmes would have fallen in love sooner, or maybe even later if he'd not considered Watson so mysterious as all that, if he'd not been watching closely to determine his character.

"Let me think," he muses, taking a thoughtful drag off his cigarette. "I would have hardly leapt on you all at once. It wouldn't be a good idea to complicate relations with a new flatmate so soon, before true characters are understood. No, I would have had to wait until I knew you better. And by then I would have fallen in love with you." He takes another drag, exhaling slowly, trying to pinpoint when exactly he'd fallen in love with Watson.

"Judging from previous experience, I would have waited until after our first couple of cases together. Perhaps I would have acted sooner, even, with the possibility of success easing the way."