Level 50 is confirmed to take 625,387 experience points.
Note: Double xp has now permanently been added. This obviously cuts the xp needed in half and makes this achievement a walk in the park.
Edit: Guardian has been reduced from 91% to 83% over the Memorial Day weekend. Annex has also been reduced down to 23%.
Edit #3: Submission has now been reduced to 47% and Wingman has been increased to 110%.
Edit #4: TU5 made a lot of changes to the conversion ratios. The new ones are located below. It also added a permanent 250 point win bonus to the mix.
The up-to-date point ratios for each gametype are as follows:
Wingman: 1.18
Warzone: 1.0
Execution: 0.98
Guardian: 0.70
Submission: 0.38
KOTH: 0.25
Annex: 0.23
If you want to get this achievement as fast as possible, use the method found here
http://www.xbox360achievements.org/forum/showthread.php?t=18...If you don't want to boost though, your best bet is Guardian or Wingman if you're good enough at it. Whatever you choose, stick with it and do your best to enjoy yourself.