Well, I don't have an exact timeframe for you, but he is pretty quick. For instance, with Ni Hao Kai Lan (if you don't know about it, it's a show like Dora that teaches Mandarin Chinese) he picked it up right away. Within 10-15 minutes after the show he was counting in Chinese and repeating the lesson. So in context, if we're in the grocery store and he sees the numbers to the aisles he'll count off in Chinese.

And after watching Mickey Mouse Clubhouse, I hear him running around saying words unique to that show that I certainly didn't teach him. Meeska Mooska Mickey Mouse and mousekatools and the like.

Oh and also... he didn't really start speaking until he was around 2 1/2. He knew about 50 words by that time. But now I'd have to sit down and make a list.