[Grammar] Preposition ‘in’ versus ‘for’

[northpath]

Recently I heard in an American movie when a teenager said:
I haven’t brushed my teeth in three days.
Instead of
I haven’t brushed my teeth for three days.
Is this acceptable?

 

[Tdol]

And it is acceptable in British English too.

 

[northpath]

So, do I really can say:
My parents have been married in thirty-nine years.
An average candle will burn in about six hours.
Hi Jack, I haven't seen you in ages.
?

 

[emsr2d2]

So, [STRIKE]do[/STRIKE] can I really [STRIKE]can[/STRIKE] say the following?

1. My parents have been married in thirty-nine years.
2. An average candle will burn in about six hours.
3. Hi Jack, I haven't seen you in ages.
[STRIKE]?[/STRIKE]

1. No.
2. Yes.
3. Yes.

 

[northpath]

Thanks, what's the rule of thumb?

 

[Roman55]

I don't think there is a rule.

My parents have been married for thirty-nine years.
My parents got married in 1977.

An average candle will burn in about six hours = that is how long it takes to be fully consumed.
An average candle will burn for about six hours = that is how long it will last.
For all practical purposes, they have the same meaning.

Hi Jack, I haven't seen you in ages = it's been ages since I saw you. (perfectly natural)
Hi Jack, I won't be seeing you again for ages, BUT
I've been waiting here for ages. (You can't use 'in')
I haven't been here in ages.