Admiral Allen was quoted by the BBC saying that even though it may just take "a couple of months" to clean up the oil slick from the surface of the Gulf--once the oil actually is stopped underwater--the "long-term issues of restoring environments and habitats will be years." Break Up of Spill Hinders Containment Efforts
Allen also noted that the break-up of the slick into multiple smaller slicks makes it more difficult to keep the oil offshore. "We are adapting to an enemy that changes. We are no longer dealing with a monolithic spill." (The Guardian)
Ecologists would likely agree with Allen's assessment that it's going to take many years before the damage to coastal and marine ecosystems is fully healed.
20 Years Later You Can Still Find Oil From Exxon Valdez
You can't make a direct comparison here with the Exxon Valdez--the conditions being so different in terms of volume of oil spilled as well as it occurring in much warmer waters (that's a good thing)--but back in January a study was released that showed how, even 20 years later, some 20,000 gallons of oil were still trapped in beaches affected by the spill.
Again, the conditions are different in this case, but that gives you some idea of the long-lasting impact that just one spill have.
More on the BP Oil Spill:
Must See Aerial Footage of BP Oil Spill Shows 'The Gulf Bleeding' (Video)
The BP Oil Spill Has Folks Singin' the Blues (Video)
Will the BP Oil Spill Be Our Collective Zen Slap Into Eco-Realization? Let's Hope So