We already know the engines can be re-used. They were tested before the launch, launched, and then relighted 3 times during the re-entry and landing phases. Now we're taling about 'service' life, or "time to overhaul", just like an aircraft engine. Consider also that after three launches, Spacex could have, on site at Cape Canaveral the basic launcher for the Falcon Heavy. All they will need is a new second stage, and a worthy payload.
Go Spacex!