Shoichiro dies from wounds inflicted by Mello soon after making the Shinigami Eyes deal. Since he didn't even use the Death Note (nor was any DN used on the people involved in the incident) prior to his death, it can be assumed that the DN did not change the lifespans of anyone indirectly involved in some killing.
So because his lifespan was halved with the eyes deal and he dies a few hours/days later (lets assume he underwent surgery and a few days seems reasonable) was his natural lifespan that much short?