Omission of 'for' with various quantified time intervals: influence of verb

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 (in §2, on p.3):

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 preposition 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

?

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

Some personal observations that won't fit in a comment box. Firstly, as a rule of thumb it seems better to use such noun phrase Adjuncts of duration (i.e. those which occur with numbers, e.g. five minutes, three days, a year) with stative verbs, verbs that describe situations and not real actions:

• I've lived here three years.
• I've been in the marines five months.
• I've had this car seven days.
• I've known Ben three months.
• I've only owned this five minutes.

These usages are more common, I believe, in American English than in British English. However, in British English they seem massively improved if there is another preposition phrase after the duration phrase. One example is the word now:

• I've lived here three years now.
• I've been in the marines five months now.
• I've had this car seven days now.
• I've known Ben three months now.
• I've owned this ring five minutes now.

Notice that we shouldn't confuse these duration Adjuncts NPs with temporal NPs functioning as the Complement of a verb. Several verbs take such NPs as a Complement:

• I waited five minutes before leaving.
• I stayed five months at that hotel.
• The film lasted three hours.

Omitting the preposition is acceptable and commonplace in the US:

I have worked here five years.

and without a direct object:

The tree has been growing five hundred years.

EDIT#1

For very short sentences or durations without a specific value, it is more common to include the for

I suffered for years.

We tried for years to get pregnant.

rather than:

I suffered years.

We tried years to get pregnant.