The conventional thinking has been that after the 28-year papacy of JP II, which began when he was 61, the cardinals made a conscious decision to pick two successive guys in their later 70s so a multi-decade papacy would be unlikely. Benedict setting the precedent of retiring will be a strong precedent for future popes to step down if their physical condition gets really marginal. Francis, who had earlier been complimentary towards Benedict's decision to retire, hedged somewhat in recent interviews, hinting he was more inclined to serve until death, or until seriously incapacitated.
I'd guess the next pope will be late-60s to mid-70s, young enough so a 15-year reign could be possible.