I am doing a 28 day Amazon Explorer on Holland America, R/T Ft. Lauderdale. We get to the mouth of the Amazon (in Brazil) on day 8. Before that , we have stops in St Maarten, St. Lucia, Tobago, and Devil's Island , Guyana.
8 days to get back, stopping in Barbados, Dominica, and Tortola.
So, if there was no dilly dallying along the way, a ship could get quite far south in 14 days. But, as mentioned, the stops along the way will be nice.
AFAIK, HAL it the only line that does that region R/T from Ft Lauderdale. They go once a year.