With respect to final reserve fuel; the JAR rule (which is pragmatic) is that if it is reached it requires an emergency call and a precautionary landing should be undertaken.
The latest ICAO Standard (not yet in place) will be that, at any time that it is anticipated that final reserve fuel will be breached before landing (i.e. when in-flight planning
indicates that condition) a 'mayday' call must immediately be made.
Whilst agreeing with the sentiment of Jayteeto's post, he has spoilt it with this statement:
If a SAFE limit is 2hrs, then it is SAFE to fly to 1:59 EVERY time
Jim