If you like freebies but don't like the idea of a whole month playing one game's multiplayer, the good news is you get 300 MSP for playing 70 hours, and 100 MSP for 35 hours. And if you only have 1500 MSP spare for Halo stuff this month, you still get 100 MSP back. You should note Microsoft says there's a maximum total of 10 million MSP on offer across all players, and they'll be doled out on a "first-come, first-serve basis."
FYI, clocking in 140 hours of Halo 4 mulitplayer by the end of the month equates to just under six hours each day. That's roughly a third of your waking life in November. To put that in persp... what's that? Oh, right. See you in December, then.