ICAO Provisions in Annex 10 Volume II
5.2.2.1.1.1 Aircraft on long over-water flights, or on flights over designated areas over which the carriage of an emergency locator transmitter is required, shall continuously guard the VHF emergency frequency 121.5 MHz, except for those periods when aircraft are carrying out communications on other VHF channels or when airborne equipment limitations or cockpit duties do not permit simultaneous guarding of two channels.
5.2.2.1.1.2 Aircraft shall continuously guard the VHF emergency frequency 121.5 MHz in areas or over routes where the possibility of interception of aircraft or other hazardous situations exist, and a requirement has been established by the appropriate authority.
5.2.2.1.1.3 Recommendation.— Aircraft on flights other than those specified in 5.2.2.1.1.1 and 5.2.2.1.1.2 should guard the emergency frequency 121.5 MHz to the extent possible.
5.2.2.1.3 Aeronautical stations shall maintain a continuous listening watch on VHF emergency channel 121.5 MHz during the hours of service of the units at which it is installed.
5.2.2.1.1.2 Aircraft shall continuously guard the VHF emergency frequency 121.5 MHz in areas or over routes where the possibility of interception of aircraft or other hazardous situations exist, and a requirement has been established by the appropriate authority.
That surely makes it mandatory everywhere.
Peter wrote:
That surely makes it mandatory everywhere.
Since 9/11 that is probably true…
In any event I think it is good practice especially on those 7 hrs flights during which you have nothing else to do anyway. 
I must admit I sometimes turn it off because the chatter interferes with my own communication.
I was monitoring 121.50 on the 4hr flight EGKA-LIPB and heard just one call: from Swiss Radar to a “lost” Montenegro-something flight. No reply was heard.
I do monitor 121.5 on longer flights and the only communication I hear is “lost” airliners 
I’ve never heard anything else except when they were asking me to respond between France and Italy when I lost comm with French ATC and didn’t get frequency of following ATC in Italy.
The big advantage of monitoring 121.5 on the COM 2 radio is that they can reach you if you ever dial in a wrong frequency.
Can an ATCO comment on how long ATC take after they realise lost comms, before they try to reach you on 121.50?
Some airlines seem to use 121.5 the same way marine VHF channel 16 is used: to ask a colleague to call them back on another frequency.
