I reached Prestige 7 today. It took me 25 hours to the minute to reach prestige 7. This represents over 2.5 hours longer than any other prestige. I carefully watched the challenges and rotated in other SMGs and Assault rifles to try and collect the Veteran 10K challenges. I also adjusted my killstreaks from 7,9 and 12 to 5,7 and 9 but I think it still affected my kills per game and therefore lowered my XP per game as well.
I think I will switch back to 3,5 and 7 and try and earn as many Veteran challenges as possible but since I have maxed out the Veteran challenges on my favorite guns its getting more difficult to earn 10K XP challenges on ideal weapons.