I've been on MS Contin (otherwise known as Codeine Contin) for more than 2 years now.
I'm taking 100mg twice a day. I once went up to 300mg a day (150mg, twice a day), for at least 6 months, maybe even a year, but since I didn't see any improvement, only more side effects, I came back to 200mg/day.
Now, the MS Contin is only helping get the pain to a lower, more manageable level, not relief it completely.