I was trying to compare someone who has allot of body fat (say 200 pounds of fat) vs someone who has relatively little (say 20 pounds). And they both got the same ratio of PUFA on them. I was thinking it would be obvious that it will take the heavier guy longer all else being the same.
The rest of what you say makes sense, but it makes me feel even more so that you cannot really estimate how long it will take to deplete PUFA because there are many individual factors.
Yeah, the person who has less body fat should be able to deplete it quicker because their RMR would be higher (due to more muscle mass) and also muscles being able to oxidize a good portion of it. But "depletion" could also mean displacement by SFA, so eating mostly SFA will change the PUFA/SFA ratio in fat even if there is not much fat loss. That is still beneficial, and can probably happen much more easily than losing the excess fat.