In the UK you will lose contact with the coastguard from the shore based stations in about 10 miles with a 5W handheld. With a 25W fixed VHF and a 3db antenna (a lot are 6db) then you can rely on good comms to the coast guard at 40 -45 miles. However, at anything over 30 miles out, then you are pretty much relying on another vessel responding to your mayday call, or another vessel providing a mayday relay.
In the last 2 years I have responded to direct Ch16 mayday calls for, a) an engine fire on a 30ft boat, b) ignorant gits running out of fuel twice at 20 miles out !, c) a fouled prop on a shaft drive, and d) this is the scariest, a chap on his own having a stroke. He made a full recovery, and we often see each other out on the wrecks.
Other times I have been called by the CG because they can see me on AIS and assume I am the nearest to respond to give a situation update.