Level 33.
wich will take another useless shitload of time.
the best is to retire at lvl 31 (because you also get 30 xp bonus when you retire, wich doesnt become more when you retire on higher lvl) once gen 16 you dont get any xp bonus anymore and you can retire at 32 or go to lvl 35 (wich will go faster with the bonus
edit:
Level 33 is 35,569,612 experience I think
if this is true and lvl 33 is only 4 mil xp more, i would retire at 33 now and than start retiring at lvl 31 from now on