There's the verb {nI'} "be long, lengthy (duration)"; so the question is "what does it take as a subject"?

A little bird told me, that there's the paq'batlh example of {Qobbogh may' nI' jun}, which shows that the {nI'} *can* take an event as a subject.

So, so far so good.. But this led me to wonder.. Could the verb {nI'} take as a subject the noun {poH}?

Yes, it absolutely can, because it has been used adjectivally: poH nI' long time (KGT).

It has been used other times:

yIn nI' yISIQ 'ej yIchep Live long and prosper (Radio Times)
A yIn is neither an event nor a time period.

nI' jajvam This day is long. (klingonska.org/canon/1997-06-29d-news.txt)
nI' ram The night is long. (KGT)
A day or a night might be an event or a time period, depending on how you look at it.

It seems clear to me that anything that might exist over a long duration, including time itself, can be described as nI'.

And if yes, then what would be the difference between {poH nI'} and {poH vItlh}?

vItlh is a very general verb that could be talking about the quantity or intensity of anything measurable; nI' is specifically about duration only. Your question is like asking what the difference is between a long time and a great amount of time. The latter only gets at its point indirectly.

-- 
SuStel
http://trimboli.name