which one is correct OR what is the difference between the following sentences:
I think the duration of the meeting would be two hours.
I think the duration of the meeting will be two hours.
As the OP is talking about a future event, the question being asked is whether to use 'will' or 'would'. 'Would' may only be used in such circumstances if there is doubt, uncertainty, or some form of impediment to the likely duration.
As it stands 'I think the meeting will last for two hours'is perfectly correct grammar. Even a small amount of doubt does not change this. e.g. I think the meeting will last for two hours, provided Charlie doesn't speak too long'.
However as soon as you introduce a word like 'but', or 'however' then it calls for 'would'. 'I think the meeting would last for 2 hours, but for the fact that Henry is sure to ramble on for an hour or more'.
You can say 'I think the meeting would last two hours, in normal circumstances', because it carries an implication of abnormal circumstances and hence of the meeting going on longer (or shorter).