Frequent...no.
Having said this, if any evidence of smoke/fumes are present, full drills are on, with a possible landing enroute a distinct possibility.
Too many folks have had problems in the past to say otherwise, IMO.
Better alive than dead, any day of the week.
Personally, only three times have smoke/fumes been a problem.
Twice in a B707, once in a TriStar.
On all three, lost sight of the First Officer (with his mask/goggles on...pronto) within ten seconds.
The Flight Engineer knew precisely what to do...and hats off to him, absolutely.