Good question, lots of variables. The temperature of the water is key, because we're talking about a biological process. Also, the effectiveness of the dispersant is critical ... the better dispersed the oil, the more surface area for the oleophilic microbes to digest.
People are discovering the oil is mostly "gone" now .. from this leak. What people don't realize is that oil leaks in the the oceans every day in significant quantities ... always has since the earth was formed ... and that's how the oleophilics were "there" to solve the problem.
The whole process of dispersants and bio tech is explained at The Patriot's Flag - Dispersants and the Piper. It is a simple process ... one that has been in place for eons and is simply enhanced by the dispersants. And much to the chagrin of the environmentalist Left ... it has worked perfectly.
See #5.
Foxnews was highlighting this Sunday morning... Markey’s accusations against BP and the Coast Guard.