"be gone for"- Grammar help

'How long will you be gone for?' sounds like perfectly normal and current American English. It's just a different way of asking 'How long will you be gone?'

"Gone" sounds OK to me.

Whether the sentence should end in "for" depends on one's opinion on preposition stranding.

http://en.m.wikipedia.org/wiki/Preposition_stranding#Opinions_for_and_against

The English language does not operate on the silly opinions of know nothings, Ori.

Just ask yourself - how can these nonsensical "rules" be part of the English language when people both before and after the "rules" were invented out of whole cloth never followed/follow them.

When you go away
As I know you must
There'll be others say
That the diction's bust

While prescriptivists
Those who won't say say not
Watch descriptivists
Get there collars hot

Who will post so long
That ripostes will be
Like your righting wrong
For eternity

When you go away
When you go away
When you go away