I came across these two examples, given to illustrate 'a case' where the inclusion of the preposition for is considered optional by the authors (Jae-Min Kim, Gil-Soon Ahn) (file:///C:/Documents%20and%20Settings/DAD/My%20Documents/Downloads/58800410%20(3).pdf)

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

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

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

Is this regional?

Do (A) the size of the DO (/locative / PP / ...) between the verb and the time phrase

(B) the actual verb used

influence acceptability?