So, the rank cap was lifted in Reach yesterday and it was perfect timing for me.  I was sitting at Lt. Col Grade 3 and after the rank lifted I needed about 10K credits to rank up to Commander.  Ended up getting that in my playing yesterday.
 
I know when I obtained Lt. Colonel - Oct 22nd - because I picked up the Make It Rain achievement that same day.  That makes 11 days between Lt. Col and Commander for me.  There is a 150K credit gain needed between those ranks, which means I've earned ~13,600 credits per day over the past week and a half.  Colonel is 250K credits above Commander, so I'll take ~18 days to rank up to Colonel if I maintain my current pace.
 
 It takes 2 million credits to reach General and at my current pace that will take me almost exactly 100 days.  My guess is I won't maintain my current pace for 100 days, but I bet I'll reach General within the first year of Halo Reach's release.
 
I tend to buy armory items that I feel give the most impact and so far that has mostly been a few firefight voices, visor colors, and a few helmet variations.  Right now I'm using Emile's helmet (etched skull on the visor) with black visor color.  There are some pieces (and I have bought a few) that use more of the secondary color or other differences I like.