Is the use of "for long" to mean "for a long time " restricted to only negative sentences and questions ? Example He couldn't tolerate it for long. Did he tolerate it for long ? Can I say he tolerated it for long?
According to CGEL*, you seem to be right about for long not being able to be used in positive declarative sentences.
(1) They didn’t laugh loudly or for long.
(2) No one laughs for long.
(3) Few people laughed for long.
(4) I doubt that he’ll laugh for long.
(5) Did they laugh for long?
(6) I’ll leave if they laugh for long.
As we see, for long does not have to be preceded by not or a negative auxiliary; it merely has to be in some sort of non-affirmative context.
*The Cambridge Grammar of the English Language by Huddleston and Pullum (Page 825).