It takes time, I would say that playing moderately It takes you one year.
Starting from Recruit it will take 100 days. Only if you hit the daily credit cap (200,000) everyday.
Took me about a year.
I did it slowly over 5
It depends on how much you will invest your time into Halo Reach. Probably at least 1 year. What rank are you currently? You can also go to HaloTracker.com, enter your gamertag, and it will show you how long approximately till Inheiritor. However, I don’t think this is completely accurate.
Since I played custom games for a total of 76 days, and 9 days of multiplayer, I am just now getting to inheritor in about a week or two. So it took me about 4 years.