Regardless of whether your pellets surface or sink, I wouldn't change how long the beer sits on them either way. Time is time, whether spent on the bottom or the top
As for the sinking part, I've used pellets with a wide range of densities, from hard as a rock and prone to sinking immediately before eventually surfacing at a raft, to very soft that would turn into a raft of pellet mush within minutes. But best as I can recall, they always eventually surface.
fwiw, I typically dry hop around 68°F and after 5-8 days start a cold crash to 34°F. By the time the beer hits terminal temperature the hops are totally sunk to the bottom...