It depends on how the drawings are used. You can easily squeeze ten seconds of footage out of three finished cels, if it's a wide pan over a couple characters talking in the distance -- which is both efficient and cinematic. And this type of shot is common in anime for precisely this reason.

Gainax is one of the past masters/worst offenders in this regard, including: a sequence near the end of Evangelion where they just shot the back of a bunch of old cels; a off-screen sex scene near the end of Evangelion where a shot of only the background plate is held for at least a minute; back-to-back clip shows in Kare Kano, where they show cel counts for each prior show to pad for time -- one episode had 2300 cels (that's total cels, not just keys).