Approximately 11,000 years according to this paper (and references therein):

Wright and Steadman (2012) Insular avian adaptations on two Neotropical continental islands. J. Biogeogr. 39(10):1891-1899

Thanks Derek!  But I meant prior to the Pleistocene...  I found a few paper suggesting that the Gulf of Paria formed between around 10 and 6 Ma, but I can't figure out when land connections were originally severed.

Sorry, can't help you there. I'd just come across that paper and skimmed it about a week ago, so it was fresh in my brain. Should've figured that'd be too easy. Good luck!