I couldn't get the difference between these?
give me a two hour leave
give me two hours leave
What is more, when it comes to using the latter, which one would you rather use?
two hours' leave
two hour leave
Both are okay. However, I'd write in this way...
Grant me a two-hour leave (note the hyphen)
Grant me two hours' leave (note the apostrophe)
That's because I am making two-hour an adjective. On the other hand, if I want to use possession, I'd go for the second sentence.
Another option could be...
Grant me a leave for two hours (clarifying all possible ambiguities in your mind!)
Give me a two-hour leave. (I've used the hyphen to make a compound adjective)
Give me two hours' leave.
An example of use:
Back at the depot she requested forty-eight hours' leave. (Millions Like Us, 2011)