Modern IRS systems have a lot of redundancy in them, as some poeple on here may have pointed out. As well as accuracy. However, errors that do occur are most likely to be "bounded" errors. This means that the errors don't get bigger as the flight goes on. Instead, the errors will oscillate from there minimum value to there maximum throughout the flight.
The most "feared" unbounded error is Wander. I want get into that, I am not in too much of the know to explain it....yet!
I think the maximum error permitted is a deviation from real track of 3nm/hr. On arrival to the gate, the IRS position error is calculated. Say the FMS says you overall position error is 4nm, and the flight time was 12hrs then this is well within limits. I doubt it would be this much though. Maybe a few metres in most cases?