'A long time' means something very different from the perspective of someone nearly 60. And the colloquial meaning of 'theory' has changed dramatically in my lifetime. It used to mean 'a suggested explanation which seems to fit the facts as I see them' and if I was writing a story set prior to November 6th 2001 I'd be very wary about having a character use it in any other way. If I was writing a story set after that date, and if the speaking character was a layman, I'd use it to mean 'a wild guess with no supporting evidence whatsoever'.