My take is along the lines of what @tuck7216 had said:
FFYL Duration is the total amount of time you have to get a second wind, towards each instance of FFYL.
FFYL Downed Time is the amount of time you were down during FFYL.
So… Each time you enter FFYL there is a specific amount of time you have before you can get a Second Wind or respawn. As many of have assumed or experienced, +50% FFYL Duration simply provides an additional 50% of whatever amount of time you were going to be given.
+50% FFYL Downed Time will add a variable amount of time to each instance of FFYL, based on the previous time you entered FFYL and got a Second Wind.
Example:
(10sec. FFYL Duration on 1st down)
+50% FFYL Duration will now provide you with a 15-second timer to get a Second Wind.
+50% FFYL Downed Time will still only provide a 10-second timer to get a Second Wind during this first time.
Now, the second time you would enter FFYL will obviously be shorter (with this effect compacting each time you enter FFYL) unless the timer to receive full FFYL, again, resets.
So let’s say the next time you entered FFYL, after being downed for 8 seconds the first time, is now 7 seconds (imagine this is already instanced in the game after receiving your Second Wind):
+50% FFYL Duration will now provide you with a 10.5-second timer to gain a Second Wind.
–Whereas–
+50% FFYL Downed Time will now give your character an 11-second timer.
(If a third time in an assumed 4-second timer after being downed for 8 seconds during the previous instance, +50% FFYL Dur. will yield a 6-second total, and +50% FFYL DT will give an 8-second total)
Disclaimers:
- not tested in-depth during gameplay.
- typed this on my phone, I apologize for any possible typos/errors
- FFYL is wonky AF and extremely variable, especially in BL3 (eventually, your Badass Tokens can help you to unlock a perk allowing to gain FFYL time during FFYL.
- regarding the above, I’m unsure if the next FFYL instance, after gaining a Second Wind, is a fixed number, or if the player would actually be given the additional Downed Time based on this perk and/or any other action skill, ability, or stat bonuses.
TL;DR -
+50% FFYL Duration does what we all know it does
+50% FFYL Downed Time gives half the time that you were previously downed into the next FFYL. (There would be no additional bonuses upon initially loading the game, unless it saves the previous time you entered FFYL… not too sure about that… or any of this tbh)
Honestly, the point is to not die lol So let’s all just do outlr best to not enter FFYL 

HAPPY LOOTING 
