since long before

English as a Second Language English Vocabulary, Grammar and Idioms
1
  1. Many factories have been losing jobs, since long before the recession hit.
  2. My son has/had a peanut allergy since long before attending public school.
  3. He had preached Christianity since long before he became a preacher.
    Please correct and comment.
    What is the exact meaning for ‘since long before’?
    Thanks.
2

"Since"should not be used in any of those sentences.

  1. Many factory workers lost jobs long before the recession hit.
    Factories don’t lose jobs, people do.
  2. My son had a peanut allergy long before attending public school.
  3. He (had) preached Christianity long before he became a preacher.

Only one of those particular scenarios has an option which can correctly take “since” with a small change:
Factories have been laying off workers since long before the recession hit. (The action started before the recession and continues now).

3

The phrase is not correct; ‘for long’, ‘long before’, ‘since then’ etc are acceptable. If you are desirous of writing your sentences with ‘since’, you may consider these but the time of occurrence of the activity will change:

  1. Many factories have been losing their employees since the recession hit them.
    (Employees may have left the factories for jobs elsewhere after the recession)
  2. My son has had a peanut allergy since he started attending public school.
  3. He has preached Christianity since he became a preacher.