Actually, that is incorrect. The GPS provides alignment to grid north, not mag north.
There are many systems, in fact most commercial ac systems, which update realtime the IRU through the GPS, most not more than a 5 minute update.(due to latency)
The kalman filters compensate between the updates. If there is not an update within 15 minutes, most newer IRS system will compensate and keep the headings.... depending on the system...but after their respective time frames, the IRS drift comes into play.
Other factors such as the maneuvers during the update timeframes...if the ac keeps a steady heading and speed, it will compensate longer...make a turn or change altitude, and the accuracy diminishes rapidly...