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*][1]" 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
1. the actual verb used

?

*Please note: The referenced paper  is very useful, but contains a few expressions that need minor corrections – possibly translation errors.*



  [1]: http://www.korling.or.kr/est/downfile.php?filename=11801985173352.pdf&filename02=58800410.pdf&PHPSESSID=19248ebe7025561156d67e76477407c8