For many years now, lets say more than 25, I have been operating SYD-LAX/SFO and return as well as SIN-LHR and return. Hence I have a fair handle on how my body copes. What I find is that at the terminal port 36 hours works best for me. Even though we have a 4 pilot crew, that is how it works for me. On the return leg I still feel "slow" and try hard to do things in a deliberate, unrushed fashion. I think we are safe enough, but I know none of us is as sharp as we could be. That's the nature of ultra long haul and one reason why we have 4 pilots.
To be honest, I don't want 48 hours - it would screw my sleeping patterns totally. The 36 hour slip works very well for me - get in, 3 or 4 hours in the cot, a walk/gym, a few beers and a feed then a good 8 -10 hours in the cot, another walk/golf etc., a couple of hours nap then off to work again.