I came across these two examples, given to illustrate 'a case' where the inclusion of the preposition for is considered optional in the paper "Acquisition of Preposition Deletion by Non-native Speakers of English" by the authors Jae-Min Kim and Gil-Soon Ahn:

a. We have lived here (for) 12 years.

b. I've studied English (for) ten years.

Though I have no problems with either version of the (a) sentence, omitting the pronoun in (b) sounds unacceptable to me.

Is this regional?

Is acceptability influenced by

1. the size of the DO (/locative / PP / ...) between the verb and the time phrase
2. the actual verb used

?